To export a notebook, long press on the notebook you want to export, then choose Export... from the dropdown menu. This will export all the notes in the notebook as PDFs (one PDF per note) and put them in a ZIP file. When the export is complete*, choose which app you would like to use to share the ZIP of PDFs.
*If there are any empty notes, password-protected notes, or failures during the export, the progress dialog will remain visible and details of the export can be viewed. To share the ZIP of PDFs that were successfully exported, tap Share.