Hi Dave,
I created a Word Document using Microsoft Word 2000, in it I wrote a sentence and highlighted the word 'test' in black so it couldn't be seen. I then used Adobe Acrobat Writer 5 to create a document of the same name, in Adobe Acrobat reader you cannot see the text that is highlighted out in black, even when you use the text tool to scroll over it, if you know what I mean.
However when in Acrobat Reader 7 I go to File > Save As Text and then look at the basic text file it has created, I can actually see the highlighted word unfortunately.
Hope that helps, ofcourse it could well be something they may have changed for newer versions. I am an IT Engineer and currently sat on one of my clients sites, but as you can see from the Word version they are still using older versions of a lot of there software.
Best I can do though in circumstances, hope it helps!