Might it be possible to have PDFs auto-update when new versions become available?
For example: most journals now publish early view versions of articles, and it is possible that changes are made between this and the final print version. The two versions usually have the same DOI. It is also common (in my field at least) for people to publish working papers that are subsequently published in journals; the two versions usually having the same title.
I guess that this might be achievable by harnessing the power of CrossMark, maybe?