Files in unsafe locations must be opened in Protected View.
This policy setting determines if files located in unsafe locations will open in Protected View. If unsafe locations have not been specified, only the "Downloaded Program Files" and "Temporary Internet Files" folders are considered unsafe locations. If enabling this policy setting, files located in unsafe locations do not open in Protected View. If disabling or not configuring this policy setting, files located in unsafe locations open in Protected View.
Set the policy value for User Configuration -> Administrative Templates -> Microsoft PowerPoint 2016 -> PowerPoint Options -> Security -> Trust Center -> Protected View "Do not open files in unsafe locations in Protected View" to "Disabled".