The Rich Text File format, RTF, was designed to allow documents containing more than just plain text to be saved in a basic file format. To this end, RTF allows documents to contain tables, shapes, coloured text, and more, and saves it all using a special tag-like format describing how it fits together.
While it is true that RTF is much, much simpler than modern word processor documents, such as Microsoft Word 2003 or OpenOffice.org, it is still able to handle the majority of documents created by users as few people really use the advanced features. The fact that RTF documents can be amended easily by hand - or by a PHP script - is a big advantage.
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