The Documents app currently operates as a subscription-based app - the purchase is handled by App Store and the subscription can be found in your iOS Settings > Subscriptions. The next billing day can also be seen there.
If you've purchased the subscription, but it doesn't seem to work for you or some of the features are still locked, please make sure that you're using the same Apple ID you'd used for the purchase. To see and manage your Apple ID, go to iOS Settings and tap your name at the top. Once you are sure that you are signed in with the same Apple ID you'd used for the purchase, go to Documents Settings > tap Restore Purchases.
Please note that according to Apple's policy, Family Sharing doesn't cover subscriptions.
If you have two devices and the subscription works only on one of them, check if they have the same Apple ID. You can also try logging out of your iCloud account/logging back in - this can be done in your iOS Settings.
If nothing seems to help, please contact us - we're happy to help you personally: