Actually, Adobe made PDF an open standard about two years ago. It's now controlled by the ISO. It's the only reason we promote it now as an acceptable open format.
That doesn't mean that Adobe doesn't still insert proprietary extensions into PDF documents using Acrobat that make them unreadable, or appear half broken by other readers. But that's Adobe's issue. They're playing the whole "We're pretending to be open while forcing you to use our closed solution" game.