I have to qualify my earlier comment. The problems arose when the files were arranged in List View (which seems to be the default, since the Files application first opens with that setting in a brand new phone). Using the Icon View instead, files open easily with one or two taps, depending on the setting chosen in “Action to Open Items”.
The “View” switch can be found at the bottom-right of the Files page (not the rightmost element there but the one next beside it).
With the “Action to Open” set to two taps (and View set to Icons), the file can be selected with one tap, and then the menu button in the top-right corner enables the various other right-click possibilities.
The “View” setting turns out to be crucial for effective operation of the file manager.