How can I reduce the photo file size on iPad?

Consider creating a shortcut within the Shortcuts App - and add the shortcut to your Share Sheet.

Here is a link to a pre-built shortcut that can resize images:

https://www.icloud.com/shortcuts/b37e70e2646941179f0a729ad2b1c230

Another option is to share the photo with yourself by mail. When you mail a photo, you can select the file size.

You can also add the photo to a shared album. Photos in Shared albums are scaled down to 2048 pixels at the longest edge. If that size reduction is sufficient for you, you can keep a selection of downsized photos in a shared album, where you can easily find them in Photos, separate from your full-sized photos in your library.

You're very welcome.

Shortcuts are a powerful tool - in this instance providing a quick and easy method of reducing image size instantly. You can simply add the installed shortcut to Share Sheet, making the shortcut instantly available system-wide.

The Shortcuts User Guide will equip you with knowledge as to how to set-up, modify and use Shortcuts with your iPad and iPhone:

Shortcuts User Guide - Apple Support

You consider creating a copy of the shortcut; here you can experiment with modifying its steps to meet a specific need without changing the original (which can be kept as a "reference"). That said, if you mess-up the shortcut, just delete it and reload a fresh copy using the link I have provided.