Hey InvalidUsernameIsAlreadyTaken,
From what we understand, you're unable to use the ruler tool in a saved PDF. If you haven't already, close the app your'e using and restart the iPad.
Quit and reopen an app on iPad
Restart your iPad
If issue persists, try a different PDF. If it happens with another PDF, test in a different app: Draw in apps with Markup on iPad These steps will help isolate the issue. If you're able to use the ruler in other apps, then the issue is likely related to either the PDF file or the app.
Make sure that the app is up-to-date: How to manually update apps on your Apple device If that doesn't help, if possible, delete and install the app:
Delete apps on your iPhone, iPad, and iPod touch
Download apps and games on your iPhone or iPad
Hope this helps. Take care.