Make it Final!
As Arlo Guthrie used to sing, this document has been
"inspected, injected, rejected, dejected and selected." When a document is marked as Final, it is saved as Read Only and it will have a security banner at the top when you open it.
Try it: Mark the Document Final
Go to
File -> Info ->Protect Document.
Select Mark as Final.

Keep going...
File -> Info ->Protect Document ->Mark As
Final