On Windows machines you can use the virtual Adobe PDF printer to "print" the pages (using the Multiple option in the Print dialog) to a single sheet and then create a new PDF file, but this printer is not available on Macintosh computers (due to a limitation enforced by Apple).
One possible workaround is to use a script to achieve a similar result. I've developed such a script and you can find it here, if you're interested:
Custom-made Adobe Scripts: Acrobat -- Print Multiple Pages Per Sheet