PPT or PDF, with a second copy in a Prezi-specific format that preserves any nonstandard stuff?

There's a lot of options of varying difficulty. But they already have code to produce PDF, and dumping some proprietary lossless representation of their internal format doesn't seem unreasonably difficult. If they're shy about their secret sauce, fine, encrypt that second copy. It's only for the case that this user wants to come back to the service later on.

I can imagine plenty of business reasons not to do this, but very few technical ones.