Make them a PDF and Acrobat has a Print Booklet feature built into the programme.
Sorry and the other way to do it is to use File>Print Booklet and set it up correctly using the dialog boxes and the Preview etc.
Then you can Print it to a PDF by selecting "PDF" from the printer option.
But I don't like printing to PDFs in general - I avoid it all costs.
Printing to PDF from print booklet on Mac is now a VERY laborious job as you need to print to Postscript, install the ADPDF9.PPD file, and distill.
For simple booklets I like Dave Saunders' 2-up impostion script: http://pdsassoc.com/downloads/Buildbooklet.zip though I haven't needed to use it for a while. It might need to run in an earlier version subfolder.
For probably any imposition you could want you can try IDImposer, a plugin that is currently free: Overview | IDImposer