PDF

Overview
PDF is Adobe's proprietary "Portable Document Format". The format preserves the exact layout of text and images put in it while allowing device-independent display and printing.

Adobe provides a free reader ("Acrobat Reader") for the format, but charges significant money for the full "Acrobat" program which can write the format. FOSS programs such as OpenOffice can export in PDF format albeit not always perfectly. There are also FOSS programs available for reading PDF -- necessary especially in the Linux world, as Adobe does not provide Acrobat Reader for Linux.

The following FOSS applications are available for reading PDFs in Linux (and they all open faster than Acrobat Reader):
 * xpdf (very bare-bones)
 * kpdf
 * KGhostView

A free/open-source utility program, pdftk, is also available for manipulating/accessing PDF files

to other formats
It seems to be somewhat more difficult to convert from PDF to other formats; some possible leads:
 * Linux CLI apps: pdftohtml pdf2dsc pdf2ps pdffonts pdfimages pdfinfo pdftoppm pdftops pdftotext
 * Perl modules: PDF

from other formats

 * Extension:WikiPDF: MediaWiki extension for displaying pages as PDF files

Annotating

 * 2008-09-10 Finally, real PDF annotating under Linux! (with help from Wine)
 * Okular has annotating features (called "reviewing"), but apparently (as of v0.8.1, 2009-03-27) cannot print annotations or merge them into the PDF for other programs to print. They can only be saved in Okular's native file format (.okular).

Reference

 * PDF articles at GnuPDF
 * PDF articles at GnuPDF

Books

 * PDF Hacks: page includes sample hacks