It's not that simple. You need a format that can represent any kind of document. Good luck trying to convert a powerpoint with complex shapes and background images into a RTF. There are only two formats that can really do that, while being viewed inside a browser - swf and images.

RTF is a good format for editable documents, but it's not such a good format for documents that need to look exactly the same on all platforms. The format that PDF actually reinvents is Postscript.