Zotero is an open source reference manager that collects all your research in a single, searchable interface. You can add PDFs, images, audio and video files, snapshots of web pages, and really anything else. Zotero automatically indexes the full-text content of your library, enabling you to find exactly what you're looking for with just a few keystrokes.
Zotero can be downloaded from https://www.zotero.org
Zotero does charge for storage exceeding 300MB. However, you can use Zotero's Preferences menu to back only your citations to the cloud instead of full-text content, which means you will never hit the storage limit. Another option is to set up Zotero to backup your citations and PDFs using DartBox, which free as long as you are affiliated with Dartmouth.
EndNote is a commercial software program that can be purchased at an academic discount through their website. It can be used to search databases, collect and organize citations and PDFs. It has an add-in for Microsoft Word with hundreds of bibliographic styles.
EndNote is expensive to purchase. There is a 30 day free trial.
Mendeley is a free reference manager that is especially strong for managing PDFs. Generate citations and bibliographies. Read, annotate, and search your PDF library. You can also securely share your papers and annotations with a project team. You can also discover other papers and other researchers who are reading the same papers as you.
There is a subscription fee for cloud storage beyond 2GB.
Papers is a PDF manager for Mac & Windows platforms. It features drag-and-drop functionality for PDFs, and allows you to search and import publications from other databases. Read, annotate, and search the PDFs in your Papers library. As with other reference managers, a Microsoft Word plugin allows you to insert references from Papers directly into your document and generate a bibliography. You can also sync your Papers library with mobile devices and home computer.
Cost: $79. A discount for students is available.
A 30-day free trial is available.