Later on it got wonky (though never even close to the extent to which Flash did!) with all the hypertextification features. But basic PDF is actually one of the Great File Formats in computer history.
Try 3D model viewer: https://youtu.be/n8KgxaNYRe4?t=27
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.
Given that nearly all reader implementations are written in C/C++, it's always going to be an easy target. Sandboxing hash helped a lot, but there's just a lot to go wrong and always will be.
PDF is still the single best/easiest format to use to render some things.
For regular use, I have come to really like SumatraPDF on Windows, it is relatively lightweight can be used without an explicit installation (hence no admin privileges are required to get it to work), and most importantly, it saves the position on opened PDF files, so if I open a file again later, I am back right where I stopped reading.