Gimposition is a small, free Windows utility which alters the internals of a PDF file to change the page sizes, orders, orientations etc.  You can get it from http://www.noliturbare.com/pdf-tools/simple-imposition.  The download is less than 1 MB, though it does require .NET Framework 2.0.

So, why would you want such a program? The most common reason would be for printing a PDF file in booklet format, so that, when the stack of printed sheets is folded, the pages of the document are in the correct order. 

The program is simple and easy to use.  Download and unzip the file, then double-click to run it..  There's no installation process required.  Type the name of an existing PDF file into the box, or drag one onto the program's user interface, and then choose your options.  The program then gets to work, creating a new PDF file in your chosen location.  It's surprisingly quick, too.  A 100-page PDF file took less than 1 second on my PC, to convert to booklet format.

Note that the program is in no way related to the freeware graphics editor the Gimp, despite the name.