I wondered about that, too, Ruth.
I know the acidity level of the adhesive side is too high for book preservation purposes and the like, but you probably don't care about the long-term preservation of the cardboard. So far, I have been unable to find anything that tells me the pH of the vinyl side of Con-Tact Paper or whether the acid from the adhesive will leech through to the front. I'll let you know if I find anything new.