You can add .pdf files directly from your computer into Mendeley Desktop.
1. Click on Add Files under File in the menu bar to choose a .pdf to add to your library.
OR
2. Simply drag the .pdf file into your library.
In both cases, Mendeley will "read" the file and create a citation.
Always check the resulting citation carefully. Mendeley sometimes may be unable to provide full citation information.
Mendeley allows you to specify folders on your computer that will be "watched" for new .pdfs.
Every time you add a .pdf to a watched folder, Mendeley will automatically add that .pdf to your Mendeley library and generate a citation.
To create a watched folder, go to File on your Mendeley Desktop menu bar. Pull down the drop down menu and click on Watch Folder. Browse your computer's files and choose a folder. Click on the box next to it. That folder will now automatically add references to your Mendeley library.