User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings
User
HKCU\software\policies\microsoft\office\16.0\common\security
This policy setting controls whether Office 2016 users can add password encryption to documents. (Users would access this feature in Microsoft Office tab--click Info, click Protect Document, then click Encrypt with Password.) If you enable this policy setting, users cannot password protect their 2016 Office documents. If you disable or do not configure this policy setting, users can encrypt their 2016 Office files with passwords.