Since the Documents app offers an option to password protect the app, there might be instances when you can't remember the passkey lock you've applied to the app.
Please note that we do not store any of your data, including logins and passwords, on our servers, so there's no way for us to remind you of your passkey - the immediate solution would be to try accessing the app by using Touch ID.
If Touch ID wasn't enabled prior to the issue, you can try accessing your data via iTunes - connect your device to a PC via USB cable > launch iTunes > click your device's icon once it appears on the iTunes' menu > search for your data and copy it to your PC.
If you're having any difficulties with the instructions, contact us at rdsupport@readdle.com - we're happy to help you personally.