Hi mel1729,
Welcome to Apple Support Communities! We're glad you've reached out for help with this behavior you're experiencing when attempting to save changes to PDF files in the Files app. Let's get to the bottom of this together.
Have you had a chance to restart your iPad since noticing this behavior? If not, please do so now, and then test to see if the issue persists. Here's a guide with the recommended steps to restart your iPad:
Restart your iPad - Apple Support
If you find that the issue persists, ensure that your iPad software is fully updated. You'll want to back up your data first:
Update your iPhone, iPad, or iPod touch - Apple Support
We're here for you if you've got any questions.
Have a great day!