Default PDF reader in the iPad pro
How to make Acrobat reader as the default app to open pdf files on the iPad Pro?
iPad Pro, 12.9-inch, 6th Gen, Wi-Fi
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The closest thing you can you is "long press" or "tap with two fingers" if using a Magic Keyboard, the PDF file and choose which app to open the file with.
I don't think you can change the default apps on a mobile device.