What you're suggesting would be amazing. For what it's worth, the popular "Skim" PDF reader on OS X stores annotations in filesystem extended attributes, so th

What you're suggesting would be amazing. For what it's worth, the popular "Skim" PDF reader on OS X stores annotations in filesystem extended attributes, so the hash of the raw file data doesn't change (but of course this comes with the drawback of more limited interoperability with other tools).