In this Adobe video tutorial the instructor shows how to compare two PDF documents. He says to go to the documents menu and choose the option to compare PDF's in Adobe Acrobat 9. This brings up a pop-up window where you can choose the files to be compared and all the various other options to choose from. This is the compare documents dialog box. Now he selects the old versions and new versions and performs other settings and shows how to perform comparisons. This video demonstrates the ability of Adobe Acrobat 9 to compare documents of different versions.