OK. Found it. I remembered working with Andy (structure) on this prior to his tragic passing.

if sys.platform == "win32":
os.startfile( helpFile )
else:
subprocess.call( [ "open", helpFile ] )

The OS test you have is fine, so that can stay.
The next question becomes "Do you care about blocking?"
I.e. the "open" call will basically just tell macOS Finder to open the file with whatever it normally would.

If you need Poser Python to wait for that app (whichever one opens the file) to quit, then a "-W" parameter after the "open" should work.

It all gets quite a bit harder if you need to test for the file being closed, rather than the app quitting.

Hope this helps. :-D

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