Portable Document Format ( PDF ) is a file format created by Adobe Systems in 1993 for document exchange. PDF is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system. Each PDF file encapsulates a complete description of a fixed-layout 2D document (and, with Acrobat 3D, embedded 3D documents using U3D) that includes the text, fonts, images, and 2D vector graphics which compose the documents.
Adobe Systems co-founder John Warnock outlined a system called "Camelot", that evolved into the Portable Document Format (PDF) file-format.
Formerly a proprietary format, PDF was officially released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO/IEC 32000-1:2008.
History
PDF's adoption in the early days of the format's history was slow. Adobe Acrobat, Adobe's suite for reading and creating PDFs, was not freely available; early versions of PDF had no support for external hyperlinks, reducing its usefulness on the World Wide Web; the additional size of the PDF document compared to plain text meant significantly longer download times over the slower modems common at the time, and rendering the files was slow on less powerful machines. Additionally, there were competing formats such as Envoy, Common Ground Digital Paper, Farallon Replica and even Adobe's own PostScript format (.ps); in those early years, the PDF file was mainly popular in desktop publishing workflow.
Adobe soon started distributing its Acrobat Reader (now Adobe Reader) program at no cost, and continued supporting the original PDF, which eventually became the de facto standard for printable documents on the web (a standard web document).
The PDF file format has changed several times and continues to evolve, as new versions of Adobe Acrobat were released. There have been nine versions of PDF with corresponding Acrobat releases:
- (1993) – PDF 1.0 / Acrobat 1.0
- (1994) – PDF 1.1 / Acrobat 2.0
- (1996) – PDF 1.2 / Acrobat 3.0
- (1999) – PDF 1.3 / Acrobat 4.0
- (2001) – PDF 1.4 / Acrobat 5.0
- (2003) – PDF 1.5 / Acrobat 6.0
- (2005) – PDF 1.6 / Acrobat 7.0
- (2006) – PDF 1.7 / Acrobat 8.0
- (2008) – PDF 1.7, Adobe Extension Level 3 / Acrobat 9.0
- (2009) – PDF 1.7, Adobe Extension Level 5 / Acrobat 9.1
The ISO 32000-1:2008 PDF open standard was published by the ISO on July 1, 2008. PDF is now a published ISO standard, titled Document management—Portable document format—Part 1: PDF 1.7
According to the ISO PDF standard abstract:
ISO 32000-1:2008 specifies a digital form for representing electronic documents to enable users to exchange and view electronic documents independent of the environment in which they were created or the environment in which they are viewed or printed. It is intended for the developer of software that creates PDF files (conforming writers), software that reads existing PDF files and interprets their contents for display and interaction (conforming readers) and PDF products that read and/or write PDF files for a variety of other purposes (conforming products).
Technical foundations
Anyone may create applications that can read and write PDF files without having to pay royalties to Adobe Systems; Adobe holds patents to PDF, but licenses them for royalty-free use in developing software complying with its PDF specification.
The PDF combines three technologies:
- A subset of the PostScript page description programming language, for generating the layout and graphics.
- A font-embedding/replacement system to allow fonts to travel with the documents.
- A structured storage system to bundle these elements and any associated content into a single file, with data compression where appropriate.
PostScript
PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. PDF is a file format, not a programming language, so that flow control commands such as
ifandloopare removed, while graphics commands such aslinetoremain.Often, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized; any files, graphics, or fonts to which the document refers also are collected; then, everything is compressed to a single file. Therefore, the entire PostScript world (fonts, layout, measurements) remains intact.
As a document format, PDF has several advantages over PostScript:
- PDF contains tokenized and interpreted results of the PostScript source code, for direct correspondence between changes to items in the PDF page description and changes to the resulting page appearance.
- PDF (from version 1.4) supports true graphic transparency; PostScript does not.
- PostScript is an imperative programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages in a PostScript document must be processed in order to determine the correct appearance of a given page, whereas each page in a PDF document is unaffected by the others. As a result, PDF viewers allow the user to quickly jump to the final pages of a long document, whereas a Postscript viewer needs to process all pages sequentially before being able to display the destination page (unless the optional PostScript Document Structuring Conventions have been carefully complied with).
Technical overview
File structure
A PDF file consists primarily of objects , of which there are eight types:
- Boolean values, representing true or false
- Numbers
- Strings
- Names
- Arrays, ordered collections of objects
- Dictionaries, collections of objects indexed by Names
- Streams, usually containing large amounts of data
- The Null object
Objects may be either direct (embedded in another object) or indirect . Indirect objects are numbered with an object number and a generation number . An index table called the xref table gives the byte offset of each indirect object from the start of the file. 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 ). Beginning with PDF version 1.5, indirect objects may also be located in special streams known as object streams . This technique reduces the size of files that have large numbers of small indirect objects and is especially useful for Tagged PDF .
There are two layouts to the PDF files—non-linear (not "optimized") and linear ("optimized"). Non-linear PDF files consume less disk space 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. Linear 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 they are written to disk in a linear (as in page order) fashion. PDF files may be optimized using Adobe Acrobat software or pdfopt, which is part of GPL Ghostscript.
Imaging model
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 1.6) 24 graphics state properties, of which some of the most important are:
- The current transformation matrix (CTM), which determines the coordinate system
- The clipping path
- The color space
- The alpha constant , which is a key component of transparency
Vector graphics
Vector graphics in PDF, as in PostScript, 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, 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 pattern of which the simplest are the radial shade (Type 2) and axial shade (Type 3).
Raster imag
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