PDF is a really useful format for conveying
information when you care about how it will
look. While HTML formatting is left to the user
agent, PDF formatting is well defined by the
document itself, so a PDF document will always
look the same, no matter what you use to view it
(depending on the quality of the viewer I
guess). Also, unlike HTML, which does not lend
itself to being printed, PDF documents
are usually ideal for printing.
The PDF document specification is available from
Adobe at
http://partners.adobe.com/asn/developer/acrosdk/docs/filefmtspecs/PDFReference.pdf,
along with a lot of other documentation about
Acrobat and PDF in the developer's section of
Adobe's website.
PDF documents can be generated from XML by
using formatting objects and a formatting
objects parser, such as the Apache project's
FOP.