power point app for my mini ipad iOS 9.3.5

You should be able to view a PowerPoint presentation without an app. However, if you wish to do anything else, you would have to purchase. The app is in the App Store, PowerPoint, but if you wish to modify, you would have to have a subscription from Microsoft. You can use your Office 365 subscription if you have one, otherwise it is read only.

I'm concerned though, whether the current PowerPoint app may require an iOS higher than iOS 9.3.5.

EDIT: I'm correct, the latest version requires iOS 10 or above.

You can download Open Office for free. It is a pretty close match to Microsoft Office. Get it here: https://www.openoffice.org/