What I like best about pdf files is that I can just give them to someone and be almost certain that any questions will be about the content rather than the format of the file.

Arrays are definitely objects. Not sure why you think objects can't have a length property nor methods like push, Object.create(Array.prototype) is a trivial counterexample of a non-array object which has these. What makes arrays special is that they are exotic objects with a custom [[DefineOwnProperty]] essential internal method, but they are still objects.