Even if the format is understood, it's also very much a case of Acrobat handling extremely badly formed PDFs instead of erroring out if it were strictly adherin

Even if the format is understood, it's also very much a case of Acrobat handling extremely badly formed PDFs instead of erroring out if it were strictly adhering. If you've ever seen a save prompt when making no changes, that happened. "It works on Acrobat" was something we got told a lot from clients sending us PDFs that were broken according to the official spec - we just had to deal with it.