Making your document read-only helps prevent changes to that copy. It doesn’t prevent someone from making a new copy of the document and saving it with a different name or in a different place, but it keeps your original file intact.
Click Review > Restrict Editing.
Under Editing restrictions, check Allow only this type of editing in the document, and make sure the list says No changes (Read only).
Click Yes, Start Enforcing Protection.
In the box that opens, type a password you’ll be able to remember, and then type the password again to confirm it.
The password’s optional. But if you don’t add a password, anyone can click Stop Protection and edit the document.