Mendeley is a free tool for managing references and PDFs through either a desktop client or an account on mendeley.com. Mendeley has the following features:
All of your research saved in one place, accessible across devices
Read and annotate saved PDFs while on the go using Mendeley's online or offline access. You can highlight, annotate, and add sticky notes.
Generate citations and a bibliography (compatible with MS Office, LibreOffice, & BibTeX)
It is social, allowing for the creation of your own citation management network
Mendeley's free version is limited to 2 GB web storage (500 MB each for personal shared storage) 3 users, and 1 shared collections (1), but users have the options for purchasing premium plans for more more features.