Follow the given steps to export your notebooks in the native ".nsa", ".pdf" or ".png" to your device's local storage-
Select the notebook or the pages to be exported
Tap on 'share' and select the desired format
Tap on "Save to Device" to save the file to your device's local storage.
At present, Noteshelf does not have an option to export your files to the external storage (SD card) due to security issues.