Mendeley Desktop not importing some files

Hello, I am trying to import files into Mendeley desktop using the "add folder" command. For some reason, all 10 pdfs in one folder and a few from another folder import but with the "broken link" symbol. The file name, however, appears at the bottom of the metadata in the window on the right (although it does not let me "open containing folder" or "open file," these options are greyed out). I am using a MacBook using OSX 10.5.8. I downloaded the files using Safari. When I tried importing one of the files on a PC with Windows 7 using Chrome to download, it worked normally. What's going on and why does the Mac have trouble? William