PDF Booklet is a free tool for creating printable booklets from existing PDFs.
At its simplest this could mean shrinking and rotating pages, then printing two on each sheet of paper which you collect together and staple in the middle.
A wide range of controls alllow you to scale, rotate or flip pages, shift them left or right, and you can apply your changes to a single page, a group (even pages, odd, all pages in that position) or all of them.
The program also offers many layout options, and these can be handy even if you've no interest in printing a file. You could create a file where every page contains 2 thumbnails of the original pages, for instance (new page 1 has old page 1 and 2, new page 2 has old pages 3 and 4, and so on).
If you're unsure what setting might work for you, just make a guess. Whatever you choose, a Preview window updates with thumbnails and overlaid page numbers to show the results.
Clicking Go creates your new file and opens it in the default PDF viewer, ready for final inspection.
Pdf Booklet's interface is basic, but there's plenty of PDF-transforming power here. Check it out.