How to unselect items?

I did a select-all in one of my folders. I can’t seem to figure out how to un-select them! If I go into a different folder and select a PDF, I see it in the preview window. If I go back to the original folder, they are still all selected! If I click on one, nothing happens. If I click in blank space, nothing happens. I even quit and then restarted DT, but the files are still all selected!! How can I do this?

When I click on one, the previous selection is undone

That should be impossible, and suggests that maybe you didn’t actually successfully quit DT at all. Please restart your Mac and report back whether the phenomenon persists.

Just as an FYI: Shift-Command-A deselects.

And a screen capture could be helpful.