The best and cheapest way to place those pages side-by-side (impose them) on an 11x17-inch page in an originating application, then create your PDF.
If that's not feasible, there are a number of third-party plug-ins that work with Acraobat and can do it for you, but at additional cost.
Randy Hagan-htech
P.O. Box 15055
Portland, ME 04112-5055
email: help@RandyHagan.com