Most books aren't available in .pdf, but you can easily take the html books, paste them into a Word document and save them as a .pdf. It takes a bit more time and effort but you don't have to worry about DRM at all. I think the boon to books in .pdf is that you can print them out if you want a hard copy on your bookshelves.
I always worry because it only take a single EMP to ruin all those digital books. I don't think that a large scale one is likely, but one should always be prepared!