Then you proactively prevent changes. Either "open a copy" or "open in readonly mode".

If you make saving the default you have to manually not save. It's a trade off versus default no saves with manual saves

Have a "Read-only" checkbox. For the love of God, have a "read-only" checkbox.

Modern apps like gdocs have a toggle for switching between read-only, suggest changes, and editing. If you forgot to toggle, you can just open version history and revert. MS Office also had version history for nearly a decade.