How to store a PowerPoint presentation in memory so,that I don’t need WiFi to open it

YOu need to store it on your device, not the cloud. If you have the powerpoint app there should be a way to store it locally.

You also need to actually have the powerpoint app on your iPad (or compatible other app, for example I think keynote will open powerpoints). You can’t use a cloud version without the internet.

One way to experiment is to either turn off your wifi or put your device in airplane mode and see what works.