Your personal Mendeley library will consist of both citations and (in Mendeley Desktop) .pdf documents.
You can add these references in a variety of ways:
Be sure to double check the information it provides, however. Mendeley can sometimes not extract complete or correct citation information.
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.
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.