I noticed that after upgrading to Win 10, Powerpoint opened all documents in compatibility mode, even if created and saved as Powerpoint 2013 documents. And when I double-clicked Word documents they opened in Wordpad.

I determined that during the Win 10 upgrade, Windows changed the file associations for Office documents. Powerpoint was for some reason changed to 'Open XML Converter.'

Changing the associations back to Office applications via Control Panel fixed the problem.