I was considering the idea.

When you hit the Share button, it'll ask you to Save the modified version and then it'll automatically export it to PDF in a temporary location and send that one.

Or ask if the user wants to also save the PDF version just sent...
Not sure.. what do you like best?

Aniello

PS: please note that when you're opening PDF with multiple pages, Xournal does process the pages in background while showing you the processed ones. This can slow down the tablet a bit and can take time.