PDF

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The Portable Document Format (PDF) is the file format created by Adobe Systems in 1993 for document exchange. PDF is used for representing two-dimensional documents in a device-independent and display resolution-independent fixed-layout document format. Each PDF file encapsulates a complete description of a 2-D document (and, with Acrobat 3-D, embedded 3-D documents) that includes the text, fonts, images, and 2-D vector graphics that compose the document.

PDF is an open standard, and is now being prepared for submission as an International Organization for Standardization (ISO) standard.

History

When the PDF first came out in the early 1990s, its general adoption was slow. At that time, the PDF-creation tools (Adobe Acrobat) and the viewing and printing software had to be bought. 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 (WordPerfect), Common Ground Digital Paper 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 free distribution of the Acrobat Reader (now Adobe Reader) program, and continued supporting the original PDF, which eventually became the de facto standard for printable documents.

Technology

Anyone may create applications that 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 sub-set of the PostScript page description programming language, for generating the page 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 (computer software) to generate an image, a process requiring many resources. PDF is a file format, not a programming language, i.e. flow control commands such as if and loop are removed, while graphics commands such as lineto remain.

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 already 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 must be processed in order to determine the correct appearance of a given page; each page in a PDF document is unaffected by the others.

Security

In 2001, PDF format attachments carrying viruses were first discovered. Virus researchers found that the PDF file viruses activated with Adobe Acrobat, but not with Acrobat Reader. As with all file formats, caution is advised. An up-to-date antivirus program is paramount.

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