If you have a collection of PDFs for your research, you may drag and drop the PDF files or folder of PDFs into your Mendeley Reference Manger window. Mendeley will extract the information from the PDF(s) and create a reference entry for each PDF file.
Note: not all PDFs have sufficient information that can be extracted by Mendeley; sometimes certain information, such as author name, journal title, or page number, may be missing, incomplete, or incorrect. You may have to enter/correct the information manually.
You can also add your PDF files to Mendeley using Add new button:
Click Add new button - select File(s) from computer,
Navigate to your PDF file(s), and add it to your Mendeley library.