When replying to an e-mail, it's useful to be able to attach a document from GoodNotes. This is different from exporting a GoodNotes document by e-mail from inside GoodNotes, which creates a new e-mail.

A possible workflow could be: receive an e-mail with a PDF attachment to review -> export the PDF to GoodNotes for signature / annotating -> reply to the e-mail and attach the annotated PDF.

Currently, it's possible to use Mail.app and GoodNotes in split screen, and drag a GoodNotes document to an e-mail reply. However, that seems to be restricted to Mail.app. Other mail apps don't work.