With drag and drop like this, I think you are inserting the PDF files as Objects and this is a form of OLE (Object Linking and Embedding) which is a windows only capability. If I'm right, you'll need to open your database on a windows system, then export and re-insert (using Insert from the Insert Menu) files before you can access them on your macs.
Many thanks. While I'm not too sharp on what that is it's the first explanation I could find and sounds like a solid answer. :)