How do I print a portfolio into a single pdf document?

I have a product manual that is 21 separate documents split into 12 different sections, and I want to be able to create a single pdf file that contains the entire document that can then be printed out (printing on both sides of the paper).  Combining the 21 documents into a single document won't work because each separate document needs to start printing on its own "front page" instead of the back page of another document.  So far, printing a portfolio won't work because printing to a pdf generates a different name for each file in the portfolio, leaving me 21 files.  My only other option is to only use single side printing, which seems to be a huge waste for a 300 page document.  Any ideas would be welcome.  Thanks!