Big Sur: PDF Service shell script cannot call python
I have discovered that Big Sur will graciously allow a script to execute in a PDF Service, if the script is put inside an Automator workflow.
Use the 'Run Shell Script' Automator action, and place the script in there.
PDF Service scripts normally take a different set of incoming arguments (title, print options, filepath); whereas the Automator workflow only passes the filepath to the PDF, so you need to modify your script accordingly.
These problems seem to be unique to Big Sur, and are fixed in Monterey, which will allow PDF Services scripts a bit more latitude.