I'd rather use Mac OS preview to view pdfs

But it doesn’t seem to be possible. I’m sure this functionality used to exist.

Anyone have a clue?

Yes, I have a clue and will share it. Via Google search “set default pdf viewer mac” I found a lot of articles. The most succinct seems to be How to Change the Default PDF Viewer on a Mac Computer

See if that works for you.

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See Preferences/General/2nd tab/Double click opens document externally (or whatever it’s called in English). This question has been asked and answered regularly here before.