The problem is not what the law requires Bob to do. The problem is what the law prohibits you from doing.
> Once it does, Bob can't stop you from distributing a version you created by buying a print copy and scanning it or taking screenshots of his DRM version and running them through OCR
"The law against breaking DRM isn't wrong but only because it is actually useless."
> Since Bob's DRM is likely used for other works not in the public domain, it's going to be hard to distinguish your breaking it for this PD work from the fact that you've broken it for lots of other in-copyright things.
That's the problem. The tools don't discriminate, so banning them goes too far and prohibits more than it is reasonable to.