Some apps such as Keynote put themselves at the topmost "layer", to prevent interruptions during the slideshow. We can turn this off, to allow Pager to appear in a layer above Keynote.
- After launching Keynote, click the "Keynote" menu, and choose "Preferences...".
- In the Preferences window, choose the "Slideshow" tab.
- Find the check mark for "Allow Mission Control, Dashboard, and others to use the screen", and make sure it is checked.