I believe what @Winston_2022 is trying to say is have you tried embedding a hardcoded image image request inside your PDF to track when the PDF is opened.
This would only provide simple, generic tracking information about the PDF (no ECID, no Campaign Info, etc)
Check out this documentation:
https://experienceleague.adobe.com/en/docs/analytics/implementation/other/hardcoded
This is also known as the no-script method.
In theory, depending on how advanced your system, I believe that PDFs can be generated on the server, and in theory you might be able to get the image embed to populate dimensions based on the campaigns, assuming the tracking part of the PDF is generated on the fly by the server… and if someone saves the PDF locally, those campaigns will be encoded….
I am not sure that is worth it (or if your company would want to go to that effort)… but I thought I should point that out in case you really need campaign data.
Good Luck