The standard is 1000 pages long. Most reader implementations are written in C/C++.
They are of course exploitable in different ways.
Adobe sometimes does not follow its own spec.
People publishing PDFs sometimes use that non-standard behavior to display some graphics. This is especially true with many research papers that only render on Adobe Reader.
In particular, other viewers often display zero-width lines, which is annoying for colormaps. Those can't safely be saved as bitmaps without oversampling either, as not all viewers can be made to avoid interpolating.
They are of course exploitable in different ways.
Adobe sometimes does not follow its own spec.
People publishing PDFs sometimes use that non-standard behavior to display some graphics. This is especially true with many research papers that only render on Adobe Reader.