Purchase and restore
From the app’s English Help & Guide · Reference release 1.0.3
Open More → Documented+ to purchase or restore the lifetime unlock.
Purchase
Tap Upgrade, review the App Store price and sharing information shown for your Apple Account, then confirm with Apple. Payment is handled by Apple.
Restore Purchases
Tap Restore Purchases if you previously bought Documented+ or received an eligible Family Sharing purchase. Restoring does not charge you again.
If it is not found
Check that the device is using the Apple Account associated with the purchase and that App Store restrictions allow in-app purchases. If Apple confirms the purchase but Documented still cannot restore it, contact support.