I agree, exporting multiple notes (entire vault or selected notes) into a single PDF would be good. I think the original poster meant generating separate pdf files for each exported notes, but maybe we could have both, and create an option for the user?
I also opened a feature request for exporting PDF files via command line. I think it could nicely pair with this feature, as you could easily state what notes you want to export on the commandline. 
But of course this feature needs a graphical user interface too, so the command line thing would not be the only way to export multiple notes.