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

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).