How do I switch between open Pages documents on my Mac?

Using a US keyboard, type ⌘ + the ` key. This key combination works in all Apple applications & most third-party apps, as well. The key combination may be different with other language keyboards.

In the first open Pages Window menu, select Merge all Windows, and all of your open Pages document windows will become tabs in Pages for rapid switching.

One method is to go to the Pages Window menu in the menu bar and select the document (near the bottom of that menu).