You have to remember that the Bigfix agent runs under SYSTEM context (in Windows, Linux etc run as root), so you need to force the pdf viewer to run in the context of the user who is to view the .pdf.
Bigfix is able to do this (see https://developer.bigfix.com/action-script/reference/execution/override.html).
There are many threads that deal with using the ‘override’ command, but this one Tip - Action Override User settings summarises things very well