An Adobe PDF ebook is more or less PDF /A with encryption. The encryption scheme is stronger than "passworded" PDFs, but has been circumvented. So you can obtain a DRM-free version of any Adobe PDF ebook you buy, and this is readable by just about any PDF Reader (since PDF/A is very basic). See Apprentice Alf's Blog for more info on DRM of all common kinds. Note that most DRM schemes for ebooks have been circumvented, and all can in principle be circumvented, so there is nothing except copyright law preventing someone from redistributing ebooks.
For more on what exactly Adobe Digital Edition PDFs can contain, see Q&A: What PDF version for eBooks?
and the ADE FAQ