Mendeley is a free* research tool that assists at every stage of the research process, from gathering research to reading and annotating PDFs to organizing citations and creating bibliographies. Mendeley replicates features that are found in other citation management tools (EndNote, RefWorks, Zotero, Academia.edu, LinkedIn, SelectedWorks, Papers, etc..) but has as its strengths: Reference Manager, PDF Organizer and Social Network.
Mendely provides a word citation plug-in, can save PDFs to your desktop/web library via a browser plug-in, and is compatible with Mac (through High Sierra as of 11/18), Windows, and Linux.
*Mendeley is a "freemium" service, which means the basic account, which provides 2 GB of storage, is free, but you can access premium features and additional storage for a monthly fee starting at 4.99/month. You can find more information on individual and team plans on the Mendeley Upgrade page.