Something positive: I had accidentally bought an e-book in PDF format from the Kobo store (I didn't pay attention, the pages still show the format for me) and contacted them to see if I could return it. They gave me a refund within a day, without trouble or questions. So at least if you do end up with a nasty PDF, they'll at least sometimes let you return it.
(I didn't expect it, since it had been my fault, but it was a nice surprise.)