Mendeley is a free reference management platform. It has both a web and desktop component, allowing you to add resources to Mendeley Web from any device and to sync to Mendeley Desktop at a later time. Mendeley is also capable of syncing with Zotero. Mendeley Desktop can be used without an internet connection.
Mendeley works very well with PDF sources, identifying and extracting basic citation information. You can highlight and annotate PDFs within Mendeley. This platform has a social networking component, allowing you to identify your own work, share resources with small groups, and connect with others.