The Codex is a vast resource for not just WordPress, but also for CSS, HTML and PHP tips and resources.
Tamba has put everything in the Codex into individual .pdf files. This makes the Codex readable and printable off line. I suggest you read the FAQ before wantonly downloading the files. The files, 244 of them weighing 34MB+/-, are listed in alphabetical order, making it easy to find just what you need.
If you find the page you need to refer to on the Codex itself, you can hop over to the list and download that page as a .pdf file, which makes it prettier in print.
PS: Tamba also suggests an Open Source software called Foxit Reader as an alternative to Acrobat Reader. I am all for Open Source!