This article describes what actions to take when your subscription isn't working.
Make sure you actually subscribed successfully and the subscription is active for your Apple ID. To check this on your device go to the Settings.app > Your Apple ID > Subscriptions. PDF Viewer should then show up like this:
If this shows up correctly then there are a few things you can try.
Reinstall the PDF Viewer app (make sure to backup your files in iCloud or other storage providers)
Restore purchases: In PDF Viewer, open subscription screen and tap on Restore purchases button. On iOS: Tap on the settings button in our app on the top left > Subscription > Restore Purchases. On Mac: Open a Document > Click on PDF Viewer on the Menu Bar > Preferences > Subscription > Restore Purchases.
Use the app for a bit and then restart it to force a re-sync. Also try restarting your device, both should force a sync.
If nothing else helps you can request a refund from Apple if a valid subscription has been made but it doesn't work. For information please take a look at Apple's documentation: https://support.apple.com/en-us/HT204084
Click here to learn more about Managing Your PDF Viewer Pro Subscription on iOS.