Just drag one or more PDFs from a dedicated folder on your desktop, share space, or USB drive into your library into Mendeley Desktop. The program then extracts the metadata (author, title, journal name, etc.) to create a library record.
Note: Keep a folder for Mendeley PDFs on each computer that you use (it only needs to have the PDFs that were uploaded from that computer, not all PDFs in Mendeley) or keep them on share space)
If the program senses that there may be errors in the record, it will be placed in a Needs Review folder until you correct the information or confirm that it is correct. Use the Search by title button to search Google Scholar, or use the research catalog entry, or PMID in PubMed to correct errors.
You can, of course, attach PDFs & other file types directly to your existing citations.