that’s pretty much what the “print” button is for that you can find in “Wppizza->orderhistory”
the output generated will depend on your settings in “WPPizza->templates->print”

i dont know much about german tax law, but presumably you need to include the values/tax rates etc related to an order, all of which can be selectively added as required in those template settings

PS: as it happens, i am also in the process of actually developing a little plugin that would allow you to selectively add a pdf invoice to any emails the system sends (the output of which is also depending on settings in wppizza->templates->print)
but that is a few days or so away yet from release