Instead of the PAPER/MODEL button (in the bottom AutoCAD status bar) and instead of mouse-double-clicking, you can also switch the paperspace and model with a keyboard shortcut.
There is no predefined AutoCAD keyboard shortcut for this, but you can add your own.
Start the CUI editor and use the "star with an asterisk" button to add a new command. Name it e.g. "PaperModel" and use the following DIESEL command as its macro:
Now go to the CUI section Keyboard shortcuts > Shortcut keys and mouse-drag here your PaperModel command from the command list. On the right in the Properties > Access > Hotkey, assign the requested shortcut to this command, e.g. Ctrl+Alt+P. Finish the CUI dialog.
Now you can toggle model/paper spaces in your layouts just by pressing Ctrl+Alt+P.
In older AutoCAD versions you can add the shortcut in the section ACCELERATORS in your .MNU file