How to add PDF file to existing astract?

Hello, I moved a bunch of abstracts over form an older version of End Note. All the abstracts imported just fine. Now, I want to be able to add the original PDF files from my hard drive to Mendeley. I tried dragging and dropping on the Mendeley abstract of the article. But, it just makes a new entry with the article. How do I add my PDF to an existing abstract? Thanks Joe