This applies to Word, Excel, and PowerPoint documents in Office 2016. Earlier versions may have different ways to protect documents.
When you want to ensure a document doesn’t get changed without your approval, adding a password provides a first-level defence. (There are ways around this constraint, though.)
Note: Make sure you have a backup copy of the document. Useful if you forget the password.
Click File > Info > Protect Document > Encrypt with Password, and enter the password. Click OK.
Removing the password is as simple as deleting what you previously entered.
Note: Besides Encrypt with Password, there is an option to Restrict Editing. This enables you to prevent new styles being created, for instance, or disallow editing.