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).

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.

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