Re: is it possible, in QGIS, to export (or save...) a map in the GeoPDF format... ?
Export to GeoPDF is not yet supported by QGIS. The new features in QGIS are user/developer/customer driven. If you want a new feature you can either develop it yourself, ask a developer you know or hire a developer.
OGR has support for GeoPDF. So a potential developer of this feature could probably make use of OGR.