[OS X TeX] TeX is TeX?

[George Gratzer]

I am writing a joint paper with HL. He uses a standard Linux TeX.

I send him the source, he makes changes, mails back the source and the  
pdf. I use Adobe Acrobat to compare his pdf with mine, to find out the  
changes made.

Result: everything changed.

So I typeset his new source and then make the comparison, and I get my  
answer.

How come his pdf is seen by Acrobat as so different. To my eye, what  
is marked up as different looks the same.

We both use TeX out of the box, same font (CM), etc.

Puzzling.

GG