Zotero is a free and open-source citation manager that integrates with many browsers for easy importation of sources. Zotero offers 30MB of free storage; additional storage for PDFs is available for a fee.

Zotero allows you to attach PDFs, notes and images to your citations; organize them into easily searchable collections for different projects; and create bibliographies using Word (for Mac or Windows). Zotero can be used as an extension with the Firefox, Chrome and Safari browsers or used as a stand-alone tool.
This link guides you to setting up and downloading the references and files you need to enhance your research.