As I couldn't find a solution to converting my simple linear lessons into pdf documents, I wrote my own perl script to do the conversion. It is based on wkhtmltopdf, which needs to be installed before using the script.
You can backup your lesson or your course and let the script do the rest of the work for you. Just run it like this...
perl lesson2pdf.pl /full/path/to/mylesson.mbz
...and, if all goes well, one or more pdf files might appear in the same folder with the script. Depending on the complexities of your lessons this might work or not work for you. So, please, don't keep your hopes too high as this is actually version 0.0.0.0.1.
If I receive feedback with the respective lesson files, I might be able to develop this further to cover more than my own needs.