In the free Mendeley version, you can add PDFs to references manually to an existing citation. Here's how to do this:

• Open Mendeley Desktop.
• Find and select the reference in your library.
• In the lefthand column, scroll down to "ADDED BY ME" and click the + after "Files".
• This opens a Windows explorer window, in which you look for the article.
• Select the article and click 'Open'.
• The article is now added to the reference.

You can also add a new citation based on a PDF:

• Open Mendeley Desktop.
• In the top left, click "+ Add references"v> "Import files from computer".
• In the Windows explorer window, select the PDF and click "Open".
• The citation will be loaded in based on the PDF. Make sure to double-check that the metadata is correct. The PDF is attached to the citation.