Here is a module that map and populate PDF forms. I'll be submitting this to the app store in the near future:
You can do it now via my "Word Document Generation for Mendix" module already available in the App store. However, the downside is that for a production environment you will also need an Exari licence. So it really depends on your budget and timescales...
I have downloaded the commons module and tried building a microflow with the overlayPDF java action in. With the pdf as the background. Not got it working yet.
Is this a suitable area to explore?