I was aware of MPDF-10, yet I believe it is a different issue.
MPDF-10 as well as http://maven.apache.org/plugins/maven-pdf-plugin/limitations.html mention the fact that menu sub-items start a new chapter.
This issue was not intended to be about pdf.xml. It aims at usage of the pdf plugin if there is only a site.xml. Menu sub-items are not appearing in the created pdf at all, even if they are listed in the site.xml.
I believe there should be a rather simple way to fix this, since this works for sub-items in pdf.xml.
Thanks for your time and consideration.