So you've bought an eBook on, say, BooksOnBoard.

And you were told it was in PDF format.

Except that it was an Adobe Digital Editions (ADE) version of a PDF, which means that you actually received/downloaded a .acsm file and that you can only open it with ADE. Once you do that, ADE generates a PDF file, which it hides under "My Digital Editions" on my computer, and that .pdf is locked with DRMs and you cannot import it on your eBook reader. If you try to open it with anything that isn't ADE, it requires a password, which you don't have, because it's fucking embedded in ADE. As far as I understand it.

So you're starting to get a little bit pissed off at DRMs. And head off to the Internets again (not that you'd ever left the Internets, but, well).