Your problem is that you have an iPad 2. If you use a retina iPad, you will be able to read a PDF in landscape if you can read a book. The screen simply becomes almost as good as print.
No, I'm saying that textbook reading on the iPad 2 is a good experience to me, aside from the rare textbook that has very narrow margins (meaning that the text is pretty small), and two-column papers.
Btw, do you have a good way to read two-column papers?
I've read many math papers and books this way.