I have achieved converting PPT to PDF using the Spire.Presentation package. See https://www.e-iceblue.com/Introduce/presentation-for-python.html

You have to pay for it: around $600 for a perpetual license. It relies on some DLL. Spire unfortunately does not work perfectly either. It left out the text box in one slide. Very strange.

I have tried using LibreOffice before, but it had a problem with EMF images.

Powerpoint automation as described in the other answers seems to work, but requires a Desktop session running; ie you cannot run it inside a scheduled task.