Portable Document Format.
A fileformat from Adobe. PDF files holds the image of any document, no matter how complex with any kind of typefaces and graphics.
The PDF file is only to bee viewed or maybe printed using the freeware plug-in Acrobat Viewer.
PDf is used on Internet and for documentation. A book delivered on a CD-ROM is a lot cheaper than the heavy paper-based hard copy.
To produce a PDF file you need a postscript print file and a program like Adobe Acrobat Destiller.