Every time amazing to see what Free Software tools are out there. This seems to be an impressive GPLv3 tool to organise PDFs in a group. It uses a web interface and you can host it on your own server.

I, Librarian on Bioinformatics

I, Librarian is a free and open source web application to build an annotated library of scientific PDF articles. It is suitable for individuals as well as for small collaborative groups, like science labs or departments.