Where Papers, iPapers, Sente and BibDesk are personal PDF managers, Librarian is a server-side application to allow groups of people to collectively build an annotate a shared PDF library that is managed from inside their Web browsers.
Librarian was designed to enable a small trusted group of researchers to create an annotated virtual library of articles in portable document format (PDF). All users may participate in the creation of the virtual library, and all users may then browse and search articles by words or phrases, much like at journal sites. The difference is that you have instant access to full text of the article, which you identified in seconds.
Metadata about each article is imported from PubMed and users can annotate articles. To get a better idea of how it works, check out the demo.
If this might be useful to you, check out the requirements, note the fact that it is free, open-source software (GPL) and give it a try.
If you try it, please be sure to let me know what you think– I may install this myself, if only because it looks neat and I’d love to start a little article-sharing database with a few friends.