You may attach files to your references, such as a PDF of the full text article. See Adding references. If a reference has a file attachment this will be indicated by an icon in the main window.
Double click on any reference with a PDF attachment, and the PDF will open in a new tab. You can then annotate the PDF:
Mendeley Desktop continues to be available but is no longer under development. For guidance on using the new Mendeley Reference Manager, Mendeley Cite and Mendeley Web, see our Mendeley guide.