You can edit PDFs that have been added to your Mendeley Library. You can do this by double-clicking to open the file in a separate tab within Mendeley Desktop. You now have the option to:
Once PDFs have been added to your Library, you can click the PDF icon or doubleclick the entry in your Library to read the documents in Mendeley Desktop with the PDF Viewer (a separate tab).
In the PDF Viewer you can: