Raziel1234/WebText-3 · Datasets at Hugging Face

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Traditionally, to go from PostScript to PDF, a source PostScript file (that is, an executable program) is used as the basis for generating PostScript-like PDF code (see, e.g., Adobe Distiller)
This is done by applying standard compiler techniques like loop unrolling, inlining and removing unused branches, resulting in code that is purely declarative and static.[19] The result is then packaged into a container format, together with all necessary dependencies for correct rendering (external files, graphics, or fonts to which the document refers), and compressed
Modern applications write to printer drivers that directly generate PDF rather than going through PostScript first.
As a document format, PDF has several advantages over PostScript:
PDF since v1.6 supports embedding of interactive 3D documents: 3D drawings can be embedded using U3D or PRC and various other data formats.[22][23][24]
A PDF file is organized using ASCII characters, except for certain elements that may have binary content
The file starts with a header containing a magic number (as a readable string) and the version of the format, for example %PDF-1.7
The format is a subset of a COS ("Carousel" Object Structure) format.[25] A COS tree file consists primarily of objects, of which there are nine types:[18]
Comments using 8-bit characters prefixed with the percent sign (%) may be inserted.
Objects may be either direct (embedded in another object) or indirect
Indirect objects are numbered with an object number and a generation number and defined between the obj and endobj keywords if residing in the document root
Beginning with PDF version 1.5, indirect objects (except other streams) may also be located in special streams known as object streams (marked /Type /ObjStm)
This technique enables non-stream objects to have standard stream filters applied to them, reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF
Object streams do not support specifying an object's generation number (other than 0).
An index table, also called the cross-reference table, is located near the end of the file and gives the byte offset of each indirect object from the start of the file.[26] This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (incremental update)
Before PDF version 1.5, the table would always be in a special ASCII format, be marked with the xref keyword, and follow the main body composed of indirect objects
Version 1.5 introduced optional cross-reference streams, which have the form of a standard stream object, possibly with filters applied
Such a stream may be used instead of the ASCII cross-reference table and contains the offsets and other information in binary format
The format is flexible in that it allows for integer width specification (using the /W array), so that for example, a document not exceeding 64 KiB in size may dedicate only 2 bytes for object offsets
To ensure backward compatibility, a hybrid-reference PDF file may include both traditional cross-reference tables and cross-reference streams, allowing older PDF processors to read the file while still taking advantage of the new features introduced in version 1.5.[27]
At the end of a PDF file is a footer containing
If a cross-reference stream is not being used, the footer is preceded by the trailer keyword followed by a dictionary containing information that would otherwise be contained in the cross-reference stream object's dictionary:
Within each page, there are one or multiple content streams that describe the text, vector and images being drawn on the page
The content stream is stack-based, similar to PostScript.[28]
There are two layouts to the PDF files: non-linearized (not "optimized") and linearized ("optimized")
Non-linearized PDF files can be smaller than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file
Linearized PDF files (also called "optimized" or "web optimized" PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since all objects required for the first page to display are optimally organized at the start of the file.[29] PDF files may be optimized using Adobe Acrobat software or QPDF.
Page dimensions are not limited by the format itself
However, Adobe Acrobat imposes a limit of 15 million by 15 million inches, or 225,000,000,000,000 square inches (145,161 km2; 56,047 sq mi), an area slightly larger than Tajikistan.[2]: 1129
The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.
PDF graphics use a device-independent Cartesian coordinate system to describe the surface of a page
A PDF page description can use a matrix to scale, rotate, or skew graphical elements
A key concept in PDF is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description
PDF has (as of version 2.0) 25 graphics state properties, of which some of the most important are:
As in PostScript, vector graphics in PDF are constructed with paths
Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text
Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves
Paths can be stroked, filled, fill then stroked, or used for clipping
Strokes and fills can use any color set in the graphics state, including patterns
PDF supports several types of patterns
The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly
This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn
Beginning with PDF 1.3 there is also a shading pattern, which draws continuously varying colors
There are seven types of shading patterns of which the simplest are the axial shading (Type 2) and radial shading (Type 3).
Raster images in PDF (called Image XObjects) are represented by dictionaries with an associated stream
The dictionary describes the properties of the image, and the stream contains the image data
(Less commonly, small raster images may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes
Image filters supported in PDF include the following general-purpose filters:
Normally all image content in a PDF is embedded in the file
But PDF allows image data to be stored in external files by the use of external streams or Alternate Images
Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these features.
Text in PDF is represented by text elements in page content streams
A text element specifies that characters should be drawn at certain positions
The characters are specified using the encoding of a selected font resource.
A font object in PDF is a description of a digital typeface
It may either describe the characteristics of a typeface, or it may include an embedded font file
The latter case is called an embedded font while the former is called an unembedded font
The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType
Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.
Fourteen typefaces, known as the standard 14 fonts or base fourteen fonts,[30] have a special significance in PDF documents:
These fonts, or suitable substitute fonts with the same metrics, should be available in most PDF readers, but they are not guaranteed to be available in the reader, and may only display correctly if the system has them installed.[31] Fonts may be substituted if they are not embedded in a PDF.
Within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding
There are several predefined encodings, including WinAnsi, MacRoman, and many encodings for East Asian languages and a font can have its own built-in encoding
(Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font's built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts).[2] The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.
For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for horizontal writing) and Identity-V (for vertical) are used
With such fonts, it is necessary to provide a ToUnicode table if semantic information about the characters is to be preserved.
A text document which is scanned to PDF without the text being recognised by optical character recognition (OCR) is an image, with no fonts or text properties.
The original imaging model of PDF was opaque, similar to PostScript, where each object drawn on the page completely replaced anything previously marked in the same location
In PDF 1.4 the imaging model was extended to allow transparency
When transparency is used, new objects interact with previously marked objects to produce blending effects
The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to PDF 1.3 and earlier specifications
As a result, files that use a small amount of transparency might be viewed acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer.
The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha
The model is closely aligned with the features of Adobe Illustrator version 9
The blend modes were based on those used by Adobe Photoshop at the time
When the PDF 1.4 specification was published, the formulas for calculating blend modes were kept secret by Adobe
They have since been published.[32]
The concept of a transparency group in PDF specification is independent of existing notions of "group" or "layer" in applications such as Adobe Illustrator
Those groupings reflect logical relationships among objects that are meaningful when editing those objects, but they are not part of the imaging model.
Logical structure and accessibility
A tagged PDF (see clause 14.8 in ISO 32000) includes document structure and semantics information to enable reliable text extraction and accessibility.[33] Technically speaking, tagged PDF is a stylized use of the format that builds on the logical structure framework introduced in PDF 1.3
Tagged PDF defines a set of standard structure types and attributes that allow page content (text, graphics, and images) to be extracted and reused for other purposes.[34]
Tagged PDF is not required in situations where a PDF file is intended only for print
Since the feature is optional, and since the rules for tagged PDF were relatively vague in ISO 32000-1, support for tagged PDF among consuming devices, including assistive technology (AT), is uneven as of 2021.[35] ISO 32000-2, however, includes an improved discussion of tagged PDF which is anticipated to facilitate further adoption.
An ISO-standardized subset of PDF specifically targeted at accessibility, PDF/UA, was first published in 2012.
Optional Content Groups (layers)
With the introduction of PDF version 1.5 (2003) came the concept of Layers
Layers, more formally known as Optional Content Groups (OCGs), refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or viewers
This capability is useful in CAD drawings, layered artwork, maps, multi-language documents, etc.
Basically, it consists of an Optional Content Properties Dictionary added to the document root
This dictionary contains an array of Optional Content Groups (OCGs), each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status (Displayed or Suppressed) of the given OCGs.
A PDF file may be encrypted, for security, in which case a password is needed to view or edit the contents
PDF 2.0 defines 256-bit AES encryption as the standard for PDF 2.0 files
The PDF Reference also defines ways that third parties can define their own encryption systems for PDF.
PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF are provided in ISO 32000-2.
PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing, or printing
These restrictions depend on the reader software to obey them, so the security they provide is limited.
The standard security provided by PDF consists of two different methods and two different passwords: a user password, which encrypts the file and prevents opening, and an owner password, which specifies operations that should be restricted even when the document is decrypted, which can include modifying, printing, or copying text and graphics out of the document, or adding or modifying text notes and AcroForm fields
The user password encrypts the file, while the owner password does not, instead relying on client software to respect these restrictions
An owner password can easily be removed by software, including some free online services.[36] Thus, the use restrictions that a document author places on a PDF document are not secure, and cannot be assured once the file is distributed; this warning is displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.

Traditionally, to go from PostScript to PDF, a source PostScript file (that is, an executable program) is used as the basis for generating PostScript-like PDF code (see, e.g., Adobe Distiller)

This is done by applying standard compiler techniques like loop unrolling, inlining and removing unused branches, resulting in code that is purely declarative and static.[19] The result is then packaged into a container format, together with all necessary dependencies for correct rendering (external files, graphics, or fonts to which the document refers), and compressed

Modern applications write to printer drivers that directly generate PDF rather than going through PostScript first.

As a document format, PDF has several advantages over PostScript:

PDF since v1.6 supports embedding of interactive 3D documents: 3D drawings can be embedded using U3D or PRC and various other data formats.[22][23][24]

A PDF file is organized using ASCII characters, except for certain elements that may have binary content

The file starts with a header containing a magic number (as a readable string) and the version of the format, for example %PDF-1.7

The format is a subset of a COS ("Carousel" Object Structure) format.[25] A COS tree file consists primarily of objects, of which there are nine types:[18]

Comments using 8-bit characters prefixed with the percent sign (%) may be inserted.

Objects may be either direct (embedded in another object) or indirect

Indirect objects are numbered with an object number and a generation number and defined between the obj and endobj keywords if residing in the document root

Beginning with PDF version 1.5, indirect objects (except other streams) may also be located in special streams known as object streams (marked /Type /ObjStm)

This technique enables non-stream objects to have standard stream filters applied to them, reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF

Object streams do not support specifying an object's generation number (other than 0).

An index table, also called the cross-reference table, is located near the end of the file and gives the byte offset of each indirect object from the start of the file.[26] This design allows for efficient random access to the objects in the file, and also allows for small changes to be made without rewriting the entire file (incremental update)

Before PDF version 1.5, the table would always be in a special ASCII format, be marked with the xref keyword, and follow the main body composed of indirect objects

Version 1.5 introduced optional cross-reference streams, which have the form of a standard stream object, possibly with filters applied

Such a stream may be used instead of the ASCII cross-reference table and contains the offsets and other information in binary format

The format is flexible in that it allows for integer width specification (using the /W array), so that for example, a document not exceeding 64 KiB in size may dedicate only 2 bytes for object offsets

To ensure backward compatibility, a hybrid-reference PDF file may include both traditional cross-reference tables and cross-reference streams, allowing older PDF processors to read the file while still taking advantage of the new features introduced in version 1.5.[27]

At the end of a PDF file is a footer containing

If a cross-reference stream is not being used, the footer is preceded by the trailer keyword followed by a dictionary containing information that would otherwise be contained in the cross-reference stream object's dictionary:

Within each page, there are one or multiple content streams that describe the text, vector and images being drawn on the page

The content stream is stack-based, similar to PostScript.[28]

There are two layouts to the PDF files: non-linearized (not "optimized") and linearized ("optimized")

Non-linearized PDF files can be smaller than their linear counterparts, though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the PDF file

Linearized PDF files (also called "optimized" or "web optimized" PDF files) are constructed in a manner that enables them to be read in a Web browser plugin without waiting for the entire file to download, since all objects required for the first page to display are optimally organized at the start of the file.[29] PDF files may be optimized using Adobe Acrobat software or QPDF.

Page dimensions are not limited by the format itself

However, Adobe Acrobat imposes a limit of 15 million by 15 million inches, or 225,000,000,000,000 square inches (145,161 km2; 56,047 sq mi), an area slightly larger than Tajikistan.[2]: 1129

The basic design of how graphics are represented in PDF is very similar to that of PostScript, except for the use of transparency, which was added in PDF 1.4.

PDF graphics use a device-independent Cartesian coordinate system to describe the surface of a page

A PDF page description can use a matrix to scale, rotate, or skew graphical elements

A key concept in PDF is that of the graphics state, which is a collection of graphical parameters that may be changed, saved, and restored by a page description

PDF has (as of version 2.0) 25 graphics state properties, of which some of the most important are:

As in PostScript, vector graphics in PDF are constructed with paths

Paths are usually composed of lines and cubic Bézier curves, but can also be constructed from the outlines of text

Unlike PostScript, PDF does not allow a single path to mix text outlines with lines and curves

Paths can be stroked, filled, fill then stroked, or used for clipping

Strokes and fills can use any color set in the graphics state, including patterns

PDF supports several types of patterns

The simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly

This may be a colored tiling pattern, with the colors specified in the pattern object, or an uncolored tiling pattern, which defers color specification to the time the pattern is drawn

Beginning with PDF 1.3 there is also a shading pattern, which draws continuously varying colors

There are seven types of shading patterns of which the simplest are the axial shading (Type 2) and radial shading (Type 3).

Raster images in PDF (called Image XObjects) are represented by dictionaries with an associated stream

The dictionary describes the properties of the image, and the stream contains the image data

(Less commonly, small raster images may be embedded directly in a page description as an inline image.) Images are typically filtered for compression purposes

Image filters supported in PDF include the following general-purpose filters:

Normally all image content in a PDF is embedded in the file

But PDF allows image data to be stored in external files by the use of external streams or Alternate Images

Standardized subsets of PDF, including PDF/A and PDF/X, prohibit these features.

Text in PDF is represented by text elements in page content streams

A text element specifies that characters should be drawn at certain positions

The characters are specified using the encoding of a selected font resource.

A font object in PDF is a description of a digital typeface

It may either describe the characteristics of a typeface, or it may include an embedded font file

The latter case is called an embedded font while the former is called an unembedded font

The font files that may be embedded are based on widely used standard digital font formats: Type 1 (and its compressed variant CFF), TrueType, and (beginning with PDF 1.6) OpenType

Additionally PDF supports the Type 3 variant in which the components of the font are described by PDF graphic operators.

Fourteen typefaces, known as the standard 14 fonts or base fourteen fonts,[30] have a special significance in PDF documents:

These fonts, or suitable substitute fonts with the same metrics, should be available in most PDF readers, but they are not guaranteed to be available in the reader, and may only display correctly if the system has them installed.[31] Fonts may be substituted if they are not embedded in a PDF.

Within text strings, characters are shown using character codes (integers) that map to glyphs in the current font using an encoding

There are several predefined encodings, including WinAnsi, MacRoman, and many encodings for East Asian languages and a font can have its own built-in encoding

(Although the WinAnsi and MacRoman encodings are derived from the historical properties of the Windows and Macintosh operating systems, fonts using these encodings work equally well on any platform.) PDF can specify a predefined encoding to use, the font's built-in encoding or provide a lookup table of differences to a predefined or built-in encoding (not recommended with TrueType fonts).[2] The encoding mechanisms in PDF were designed for Type 1 fonts, and the rules for applying them to TrueType fonts are complex.

For large fonts or fonts with non-standard glyphs, the special encodings Identity-H (for horizontal writing) and Identity-V (for vertical) are used

With such fonts, it is necessary to provide a ToUnicode table if semantic information about the characters is to be preserved.

A text document which is scanned to PDF without the text being recognised by optical character recognition (OCR) is an image, with no fonts or text properties.

The original imaging model of PDF was opaque, similar to PostScript, where each object drawn on the page completely replaced anything previously marked in the same location

In PDF 1.4 the imaging model was extended to allow transparency

When transparency is used, new objects interact with previously marked objects to produce blending effects

The addition of transparency to PDF was done by means of new extensions that were designed to be ignored in products written to PDF 1.3 and earlier specifications

As a result, files that use a small amount of transparency might be viewed acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer.

The transparency extensions are based on the key concepts of transparency groups, blending modes, shape, and alpha

The model is closely aligned with the features of Adobe Illustrator version 9

The blend modes were based on those used by Adobe Photoshop at the time

When the PDF 1.4 specification was published, the formulas for calculating blend modes were kept secret by Adobe

They have since been published.[32]

The concept of a transparency group in PDF specification is independent of existing notions of "group" or "layer" in applications such as Adobe Illustrator

Those groupings reflect logical relationships among objects that are meaningful when editing those objects, but they are not part of the imaging model.

Logical structure and accessibility

A tagged PDF (see clause 14.8 in ISO 32000) includes document structure and semantics information to enable reliable text extraction and accessibility.[33] Technically speaking, tagged PDF is a stylized use of the format that builds on the logical structure framework introduced in PDF 1.3

Tagged PDF defines a set of standard structure types and attributes that allow page content (text, graphics, and images) to be extracted and reused for other purposes.[34]

Tagged PDF is not required in situations where a PDF file is intended only for print

Since the feature is optional, and since the rules for tagged PDF were relatively vague in ISO 32000-1, support for tagged PDF among consuming devices, including assistive technology (AT), is uneven as of 2021.[35] ISO 32000-2, however, includes an improved discussion of tagged PDF which is anticipated to facilitate further adoption.

An ISO-standardized subset of PDF specifically targeted at accessibility, PDF/UA, was first published in 2012.

Optional Content Groups (layers)

With the introduction of PDF version 1.5 (2003) came the concept of Layers

Layers, more formally known as Optional Content Groups (OCGs), refer to sections of content in a PDF document that can be selectively viewed or hidden by document authors or viewers

This capability is useful in CAD drawings, layered artwork, maps, multi-language documents, etc.

Basically, it consists of an Optional Content Properties Dictionary added to the document root

This dictionary contains an array of Optional Content Groups (OCGs), each describing a set of information and each of which may be individually displayed or suppressed, plus a set of Optional Content Configuration Dictionaries, which give the status (Displayed or Suppressed) of the given OCGs.

A PDF file may be encrypted, for security, in which case a password is needed to view or edit the contents

PDF 2.0 defines 256-bit AES encryption as the standard for PDF 2.0 files

The PDF Reference also defines ways that third parties can define their own encryption systems for PDF.

PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF are provided in ISO 32000-2.

PDF files may also contain embedded DRM restrictions that provide further controls that limit copying, editing, or printing

These restrictions depend on the reader software to obey them, so the security they provide is limited.

The standard security provided by PDF consists of two different methods and two different passwords: a user password, which encrypts the file and prevents opening, and an owner password, which specifies operations that should be restricted even when the document is decrypted, which can include modifying, printing, or copying text and graphics out of the document, or adding or modifying text notes and AcroForm fields

The user password encrypts the file, while the owner password does not, instead relying on client software to respect these restrictions

An owner password can easily be removed by software, including some free online services.[36] Thus, the use restrictions that a document author places on a PDF document are not secure, and cannot be assured once the file is distributed; this warning is displayed when applying such restrictions using Adobe Acrobat software to create or edit PDF files.