Another option is to explicitly start in read-only mode (modification buttons hidden / grayed out, some distinct mode indicator "Viewing Document" next to a but

Another option is to explicitly start in read-only mode (modification buttons hidden / grayed out, some distinct mode indicator "Viewing Document" next to a button to "Start Editing", etc), and when the user chooses then switch into autosave mode. At this point many users are used to autosave and don't pay due attention to the document saved state. With Microsoft Windows' habit of rebooting your system overnight without your explicit permission, I'm concerned that this might lead to a lot of lost work.