Wow, this thread got seriously modsmacked!
I'll repeat it here that MOST of the UofP PDF's are DRM locked and ONLY usable using the Adobe Reader software application, which isn't available on the iPad or any other i-Device for that matter, and likely never will be given the Apple/Adobe warfare.
This is a fact that has been discussed many times on a number of forums (easily proven via a Google search), confirmed on the official Apple support forums AND confirmed by UofP's tech support group.
Some of UofP's PDF's are NOT locked and these should work fine. Those that are, you are SOL if you try to access them via the iPad.