If you are unable to edit existing documents and notes on your iPhone 6S, please use the following workaround until Notebooks 8.1.3 will be released on the App Store:
To edit an existing document, tap the text very quickly, just like touching a hot surface. This should bring up the keyboard.
The update is waiting to be released by Apple.
Having Issues with Notebooks 8.1.2 on your iPhone 6S?