Most people find GoodReader to be a worthwhile addition to their iPad. It does an excellent job of reading PDF files, and you can visit websites to download them from inside app. It also adds itself to the Open In list of apps for downloads, making it possible to copy PDFs from Mail or Safari.
It also does a few dozen other useful things.
If you want a more book like reading experience, try the iBook app. It's free, and like GoodReader will show up in the Open In options.
Leelai gave you a link to the iPad User Guide as a pdf file. You can also download it for free from the iBook Store (found in the iBook app). It will tell you how to use Open In and other features of the iPad. I like having the PDF on my computer and the iBook version on the iPad. Handy.