I've been using http://www.qiqqa.com/ for 8 years and it does a fantastic job of organizing my PDFs and managing citations. It can do cloud based shared libraries as well.
for me the killer feature is that it makes all the PDFs full text searchable so it is like my own personal google where the links never break and the content is all relevant.
great! good to have an alternative to qiqqa in case it disappears, although it looks like qiqqa has killed their cloud libraries and gone open source. I found qiqqa after google desktop was EOLed.
I have been quick to recommend Qiqqa whenever it seems like it might be a useful tool but I never really saw those comments get much traction - maybe because everyone was already using zotero?
for me the killer feature is that it makes all the PDFs full text searchable so it is like my own personal google where the links never break and the content is all relevant.
Highly recommend!