About the .PDF File Extension
While many document standards have appeared over the years, Adobe Systems' "PDF" or "Portable Document Format" has achieved an unheard of level of acceptance and popularity. Adobe introduced this format in 1993 and made the PDF reader software a freely downloadable utility several years later, which increased the rate at which the format was adopted.
Today, many companies and non-profit organizations have adopted PDF files as their de-facto standard for document management. They're used to distribute both internal and customer documents such as user manuals, forms, and even whole books. Organizations usually ship the PDF files on installation CDs or make them available via the Web, along with a pointer to Adobe's own website.
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PDF files can come in many forms. Initially, the documents were effectively per-page images and therefore could not be searched for text strings or other information. Later versions of the PDF format added full-text search functions, markup capabilities (like highlighting and annotation), and other features that made the documents more flexible. PDF is not an "editable" format. It's an output format meant for final document revisions that no longer require changes.
PDF Reading and Writing
Another feature of the PDF architecture is that it lends itself to accessibility concerns. Many applications, such as JAWS and various Kurzweil programs, can provide screen reading capabilities for PDF files. This makes them easily accessible to visually challenged users.
The PDF format itself was made open standard by Adobe. This means anyone can write applications that use it. Adobe maintains ownership and holds patents on the format. Many applications can write PDF files: Adobe's own Acrobat software, various document scanning utilities, and certain word processors can create PDF output files. A number of public domain PDF reader/writer applications are also available.
PDF files have occasionally been a vector for computer viruses, though this is uncommon. In 2001 a virus known as OUTLOOK.PDFWorm" or "Peachy" appeared, but no further reports of PDF-borne viruses have been noted.