Thank you for your post.
I am having difficulty understanding how the PDF file was stored.
If the PDF file is stored as a reference, then make sure that reference is available to other users. Have each user mount the volume prior to opening the FileMaker file.
If the PDF file is stored as a picture with a Mac, the image will display on all clients. If loaded on Windows, it will only display as an icon, and you will have to export it to display the contents.