If the goal is open a PDF file that currently exists on the host server so you can view the contents then when running on WindX, you need to send a copy of the file to the workstation, then you can use SYSTEM_HELP to launch the default PDF viewer.
Now technically if the PDF file on the host is in a location on the server that can be directly accessed using a UNC, then you can just pass the UNC to the SYSTEM_HELP directive.