For over 4 years, I have been successfully using the Askubuntu method:
How can I export a PDF from GIMP with each layer on a new page
on LTS 14.04 to export an mng file and then convert to pdf. However, as of sometime between Sep. 19 and Oct. 26, the convert part no longer work and produces an error message that I do not understand:
drrm@drrmlinux2:~/Downloads$ convert vt-02017_markup.mng vt-02017_markup.pdf
convert.im6: not authorized `vt-02017_markup.pdf' @ error/constitute.c/WriteImage/1072.
Can anyone help?