.pdf is the standard file extension for document files created with Adobe Acrobat software. As .pdf files have reached a "de facto" status as WYSIWYG documents, they are used extremely lot across the world for publishing manuals, letterheads, documentations, etc.
.pdf files can be viewed easily with Adobe's free Acrobat Reader software, but the "full-blown" Acrobat that is used to make .pdf files, is a commercial software. However, several tools, including open source office suite OpenOffice.org, support .pdf creation from documents.
