The PDF spec is well designed to be both very expandable and easily readable.

It's absolutely horrible to try and edit.

That's because the structure of a PDF is essentially a bunch of media "streams". It's very easy to say "render a jpeg at this location on the page" but that's about it. It doesn't store, for example, the fact that you might need to wrap words around a page. Instead, it's "Here's a box with text in it".

The only thing that really could make PDF rendering hard is adobe put a whole bunch of garbage into the spec. For example, the full spec had the ability to run javascript and flash at one point (not sure if it does anymore).

JavaScript must still be there, because I think that's how form validation works? Don't recall Flash ever being there though.

It was acrobat reader, but not in the standard. For some time only the reader was needed and not the player.

No. There are hundreds of programs that easily read and create PDFs. OTOH, reading .docx is a pain. Far be it from me to defend Adobe, but PDF is nothing like MS Office formats.

Ooh that’s not fair. Many PDFs don’t conform to the spec and how Acrobat processes them is completely undocumented.

It was funny for a few years when Apple just released Preview, and it rendered many PDFs better than Adobe's own Acrobat

Yeah it's a great app, one of the big reasons I still use MacOS - but there's certain features which you still need Adobe Acrobat for, I know to fill in the tax forms for my country you do.

I mean the rendering part. AFAIK there are no that many renderers.

No, that spec is outright nice if you consider the PCs it was made for.