Refry the pages back through the Adobe PDF printer.
Open the PDF in question.
File | Print > Adobe PDF printer. Make sure you have all the defaults set. Especially 'Shrink oversized pages' - You need to make sure you are targeting 8.5x11 in both the File | Print dialog as well as the printer's properties.
Let 'er rip.
A 'poor person's imposition tool is easy as well... same thing, only this time 'Multiple' - where you are putting multiple images on the same page.
You can also test the poster and booklet tools before wasting a ream of paper. Or even create the necessary PDF so that someone with Reader can just print them off.
There are a lot of manipulation and PDF corrections that can be done by passing a PDF back through the Adobe PDF printer.
For quick one-off items the Adobe PDF printer can be a lifesaver. If you are doing this in a production manner - go buy the professional tools - such as Quite Imposing Plus.
Warnings - You do have to check your fonts, colors, and images for undo manipulation. I'm not saying to not refry - I'm saying to check the output (and correct if necessary) before sending it off for production.
Doug