This is ABSOLUTELY a guess, but I have the feeling it might have to do with font embedding. I know that Preview doesn't embed the fonts into a pdf when it saves as pdf a document it just created (from another app's print dialog). That said, I have no idea what would happen if it were to try to save a pdf document that contained a font that wasn't installed: I doubt it would embed the font now, nor would it probably give a warning. Try downloading the pdf again and see if the original is the same size as the one you changed. Again, just a guess.