A change in Gnome3: the old way of assigning keyboard shortcuts in applications does not work by default, but the change is small. Yet again, it is a question of enabing the correct option. Open dconf-editor. Open group “org→gnome→desktop→interface”. Find key “can-change-accels” and put a mark beside it. Close the program.
Now you can, for example, open Evince and test this. Open a document, too. Then, open the menu “View” and move the mouse over “Best fit” so that it changes color. Do not press the mouse button. Instead, press “Shift+Z”, and you’ll see “Shift+Z” appear beside the text “Best fit”. Move the mouse away from the menu and close the meny by clicking somewhere else. Now you can test your new shortcut by pressing “Shift+Z”, and the document should be zoomed so that the whole page is shown.
This should work in other Gnome programs, too.