Mendeley

Mendeley is a free cross-platform application that helps researchers manage and annotate their research paper collections. Researchers also can set up collaborative projects, work and discuss in groups, and share data on Mendeley’s web platform. Downloading the Mendeley application provides tools for Word (including Word for Mac) and LibreOffice. Mendeley is particularly popular in scientific communities. 

Mendeley's website offers a good selection of user guides and tutorials to help you get started.

Even if you don't download Mendeley, you can use their free online library of research papers to search for items in your subject area. These library records indicate how many researchers are reading or have saved a paper to their own Mendeley library.