We have that in Notable PDF ( http://notablepdf.com http://notablepdf.com ). We don't require that you host the file with us either - Files are fingerprinted, a

We have that in Notable PDF ( http://notablepdf.com http://notablepdf.com ). We don't require that you host the file with us either - Files are fingerprinted, and comments can be made attached to that fingerprint (private by default, but can be made public to anyone who has the file). We're focusing on annotating documents within teams, schools, etc, but the public annotations to improve documents (à la rapgenius) is something we are thinking about. (We actually have the ability to share files with a link too)