In Fullscreen, making header bar semi-opaque and hide whenever possible; add button to leave fullscreen mode
One of the main reasons for having fullscreen mode is to allow someone to maximise the amount of screen space used to display the document. GNOME's design pattern for fullscreen helps to achieve this by making the header bar semi-opaque and hiding it whenever possible. It would be great to do that in evince!
Another thing that's missing in fullscreen is a clear and obvious button to leave fullscreen mode. We typically do this by replacing the window close button with exit fullscreen.