If I add a note in Acrobat using the typewriter tool, save the pdf and open it with Xournal, the typewriter note is displayed perfectly but will not be exported. This behaviour does not seem to be depending on the poppler version used (0.12.4 and 0.16.7 produce the same result). Is there any way to avoid that?
Using the print-to-pdf-option, produces the correct result, i.e. including the Acrobat typewriter note.
I have to add that ALL Acrobat comment tools are affected but the print-to-pdf-option solves the problem.
Indeed - the code for File -> Export to PDF does not rely on the poppler library, and is not aware of annotations (in the sense of Acrobat) in the PDF file. Whereas the code for displaying PDF backgrounds on the screen and for File -> Print -> Print to file, which relies on the poppler library, is aware of annotations, so they get displayed and printed properly.
The code for File -> Export to PDF is legacy code and, given the effort needed, the easy fix (use file -> print, as you noted yourself), and short amount of time remaining before Xournal is made obsolete by Xournal++, I don't think I'll have the energy to fix it.