> Suppose Bob was the copyright owner, last year, before the work entered the public domain. He never distributed any copy without DRM, so no DRM-free copies exist. This is fine? Section 1201 of the DMCA was created to eliminate the public domain?

No law compels Bob to provide people with new copies once the work enters public domain (and remember, Bob's publication might contain separate works with their own copyrights like cover art or an introduction). 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 (as long as you don't include the cover art or introduction). If Bob's DRM was unique to this one book, there might be an argument that breaking it was the same (as long as there's no cover art and no introduction). 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.