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Free readers for many platforms are available for download from the Adobe website, and there are several free open source readers, including Xpdf for POSIX-like systems with the X Window System; GPdf, a derivative of Xpdf for GNOME; GSPdf and ViewPDF, for GNUstep; and front-ends for many platforms to Ghostscript, q.v.
PDF is primarily the combination of three technologies:
PDF is a subset of those PostScript language elements that define the graphics, and only requires a very simple interpreter. For instance, flow control commands like if and loop are removed, while graphics commands such as lineto remain.
That means that the process of turning PDF back into a graphic is a matter of simply reading the description, rather than running a program in the PS interpreter. However the entire PS world in terms of fonts, layout and measurement remains intact.
Often the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that the PS code outputs are collected and tokenizeTokenizing is the operation of splitting up a string of characters into a set of tokens. The term is also used when during the parsing of source code of some programming languages, the symbols are converted into another format that is much smaller. Most Bd, any files, graphics or fonts the document references are also collected, and then everything is compressed into a single file.
As a document format, PDF has several advantages over PostScript. One is that a document resides in a single file, whereas the same document in PostScript may span multiple files (graphics, etc.) and probably occupies more space. In addition, PDF contains already-interpreted results of the PostScript source code, so it is less computation-intensive and faster to open, and there is a more direct correspondence between changes to items in the PDF page description and changes to the resulting appearance of the page. Finally, if displayed with Adobe Reader, a font-substitution strategy ensures the document will be readable even if the end-user does not have the "proper" fonts installed.
When PDF first came out, in the early 1990s, it was slow to catch on. At the time, not only did the only PDF creation tools of the time ( AcrobatAdobe Acrobat was the first software to support Adobe Systems' Portable Document Format. It is mostly described in those entries. The Acrobat Reader program (now just called Adobe Reader) is available as a no-charge download from Adobe's web site, and all) cost money, but so did the software to view and print PDF files. Additionally, there were competing formats. Adobe started distributing the Acrobat Reader program at no cost, and continued to support PDF through its slow multi-year ramp-up. Competing formats eventually died out, and PDF became a well-accepted standard.
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