I recommend printing the reports with PDFCreator as an PNG Image, you can then use a graphics program like Paint .NET to make the background transparent and layer both reports on top of each other.
Using some color transformation on one or both of the images (e.g. color one in red, the other in blue) it should show you to see the differences very good.
You find PDFCreator here http://de.pdfforge.org/pdfcreator. It's completely free to use.
