It makes `maven-pdf-plugin` look a bit more like `maven-site-plugin` in that it adds a `pdf:stage` goal which aggregates the intermediate outputs of `pdf:pdf` of each sub-module.
It is a working solution (at least in another project I work on) but nothing is perfect. TOC numbering seems wrong in final PDF (maybe not using a recent doxia with closed [DOXIA-564](https://issues.apache.org/jira/browse/DOXIA-564); dependency issue on my part?).
I used JSON to serialize/deserialize the TOC of each sub-module but I can only start to guess it isn't standard maven way.
Comments are welcome.
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