Device independent

What does 'device independent' mean. Why is something like a PDF, JPEG, TIFF file device independent but a microsoft word file is not? I have looked these things up but the language to describe it is really circular and not very clear.

I think they really mean "platform independent" - i.e. not restricted to a specific system, like the Mac, or Windows.

That said, I'd say that PDF was almost as locked in as DOC.