The simplest solution for navigation would be to have a separate harddrive or partition to store the PDFs.
HD (named Data?) > pdfs
Your client machines must be able to mount the drive/directory where you've stored the PDF files. Mac and windows systems map shared directories--such as the one where you are storing your PDFs--differently. You can see the differences by creating a calculation field that returns text where you just enter the name of the container field as its sole term. The third line of text in this field will show the filepath stored when a file was inserted into the container field.
You can then insert a file one the Mac in one record and insert the same file from the windows machine in another record to see how the file paths differ.
You can either use two different container fields--one for mac and one for windows or you can write a calculation field that computes the correct filepath for the current user's platform set to return "container" as its field type.