Please note that Mendeley's Desktop app does not currently work with OS Catalina. If your computer is running Catalina, you can access and use your library at mendeley.com/library.
Mendeley is a free software that will help you manage your research, annotate PDFs, and format citations.
Mendeley has both a cloud-based and desktop component. First, you will create an account with Mendeley, and then once you've downloaded the Mendeley Desktop software, it will prompt you to log in with your account. There are also free mobile apps available for both iOS and Android.