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.