You can use VPMAX  (same as double-click within a viewport layout) or VPMIN (same as double-click outside a viewport layout) to switch within model space and paper space tab of a layout.
VPMAX expands the current viewport for editing and VPMIN restores the current layout viewport.
The advantage of using these two commands is that you can pan and zoom while you are in model space, and when you go back to paper space the position and scale of the objects in the layout viewport remain unchanged.
It is useful when you zoom in a viewport on paper space while your viewport borders are not visible and accidentally double click inside the viewport and you cannot double click out.