I'd recommend using the UNC path to the file (i,e.: \\Computer\Path\To\File). It should work on all of the clients, and may work on the hosting server as well. Give it a try with a sample record. If it does not work on the hosting computer, leave off the first portion (i.e.: \\Computer\) in the element/field, and then attach the appropriate computer name on the clients dynamically in SBasic. That way the remote client will have \\Computer\Path\To\File and the hosting computer will have just \Path\To\File. To distinguish one computer from another you can use the "-user_param" flag on startup and the @UserParameter function in SBasic, if needed.

Whenever possible, always leave off the drive letter, if you can. It is a archaic leftover from CPM/DOS and was always a bad idea, even then. File systems should be a uniform hierarchy with one root directory, even if spread across several hardware platters.

If UNC doesn't serve, I'd recommend using one of the Window's implementations of NFS. This will allow you to mount another computer's file-system as a directory on the local computer. You could then create a directory on the server named (for example), "Images" and then mount that directory on all of the client computers also simply as "Images". That would allow you to use the same path on all the computers, without having to specify anything as clunky as drive letters or computer names.

Several of the cloud implementations mount as either drive letters (bad) or as UNC paths. Some use a scheme similar to NFS where a particular path / directory represents the starting point of the cloud storage. If you intend to host the cloud locally, this may be a viable and preferred option. Using a remote cloud through the Internet may be cumbersome if your PDF files are large or your Internet connection is either slower or less reliable. Not many of us have 100 Mbs Internet.