[hnick]

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.

[layer8]

Yes, and the same is true for MS Word. "Undocumented implementation algorithms" was meant to subsume that aspect.