I just installed the PDF Reader from YUYAO Mobile Software, Inc., on my iPad (available for $1.99 from the Apple App Store). I was able to easily connect to my box.net account and quickly download multiple PDF files to my iPad.
One of the books I was looking at has a print trim size of 8.5″ x 11″. It is very readable and looks great on the iPad. I wouldn’t want to read this book in PDF format on an iPhone, but it’s great to be able to view books in their printed form on the iPad.