The Portable Document Format, or PDF format, is a variant of postscript meant to be viewable on screen across platforms. It was invented by the wonderful folks at Adobe. There are many viewers available for various platforms, APDF being the most popular one for the Amiga platform. On most other plaforms you can use Adobe's own viewer, Acrobat Reader. Adobe Acrobat is the application that most non-amiga people use to create complex PDF documents.
Happy viewing. :-)