7/2/11: I only rarely use PDF files on my iPod, and my focus to date has been on those typically having 2 or 3 columns in portrait orientation. PDF files having no columns will typically need to be read in landscape orientation for the text to be big enough to read; I'm not sure yet how much I will eventually cover that type of usage. I have also since loaded more PDF-reading apps on my iPod, but to date I have looked at those even less than the few I list below.
- Dropbox (whose main functionality is to allow sharing of files over the web after you make an account, but which has a built-in PDF reader)
- PDF Reader Lite by Kdan Mobile Software LTD (my favorite)