Thanks, I wonder who is the idiot who downvotes an honest question.
Still, I would pay $9.99 for mobi or epub. PDF is inconvenient for the Kindle and the iPad. Converting with Calibre is a chore, and rarely yields good results.
I find GoodReader excellent for pdfs on the iPad. It's not only a great reader but also allows annotation etc plus makes getting pdfs into and out of your iPad fairly painless