file-jbig2pdf has only been tested on Linux. It requires [jbig2enc](https://github.com/agl/jbig2enc) to be installed, and its *jbig2* command to be available on PATH.
Install *file-jbig2pdf.py* and *jbig2topdf_lossless.py* under *$HOME/.config/GIMP/3.0/plug-ins/file-jbig2pdf/* and ensure that *file-jbig2pdf.py* is executable.
Set up your GIMP image as desired in the standard fashion for PDF export (one layer per page, bottom layer first) and under *File* → *Export As…*, expand *Select File Type (By Extension)* and select *Portable Document Format (JBIG2)*.