Word - Comparing Documents for Changes - Meet the "Compare" button

Am I the last person on the planet to know this function was pre-built into Microsoft Word 2010?

I've been using a free application called Winmerge to compare documents for changes between one version and another.  You can read more about it at http://www.codejacked.com/comparing-two-versions-of-a-word-document/.  It works awesome for comparing two code files side by side and seeing the changes between them.  Problem is, I can't figure out how to get it to work with Word 2010 .docx files, it gives me a bunch of gobbledy gook instead of words.

Enter the "Compare" button in Word 2010.

It's under the "Review" tab in the top menu of word.  Click it and you'll be prompted to select two Word documents.  It then lists all the changes between these two versions and shows the two docs as well as you scroll through the changes.

I'm using this to review a contract before signing it, just to make sure there haven't been any changes slipped in since I last reviewed it.

Now, I just need to figure out how to compare .pdf files...