You can download the full PDF Reference (PDF: 4.3 MB / 696 pages)(second edition) as a PDF file
(book normally published by Addison Wesley) from Adobe and there are even more documents about that available in theirDeveloper Program.

PDF is using the "way cool" plain text format to stay portable, so it's easy to process :-)
e.g. you can view and edit the contents of any PDF file with a text editor.

To convert HTML to PDF you could use the pdflib which is a commercial Code library with function wrappers for perl java and C++ and .. BUT thats some bucks to pay (for commercial use). The Author also wrote a book about PDF and as well how to use his library.
But still you need to translate the HTML to the appropriete PDF descriptors.
An Idea here seems to be parsing existent HTML but to go on the reight side it seems much better to use XML as a BASE and produce HTML and PDF from it.
Thats the way I do but as I don't own the rights of the code produced I cant post that. sorry
On CPAN you'll find some modules when searching for PDF, as merlyn suggested.

There are also some packages around on the net that support a wide range of PDF, e.g. search C|NET or other scripting archives. There are a few - just search.

Have a nice day
All decision is left to your taste

UPDATE also interesting is first alpha release (v1.0a24) of PDF on the Fly, a Perl5 library for creating PDF files