Whenever you need to open your file in Documents to work with it or you simply want to make sure your file is stored there safe, you can send a copy of the file to the Documents app from other apps using the Open in dialogue.
The steps are listed in order, so start with the first one, see if that helps, and then continue to the next one if it doesn't.
If the Documents app is not among the available apps, please check the 'More' button:
Check if the latest iOS version is in use on your device since the way the extensions work relies on the system:
You can also save files to a specific folder in the Documents app (e.g. when saving the PDF file from Safari) using the Files app. For this follow the steps below: