You can do so, but there are limitations.
A word document is an advanced tool of microsoft which provides ample ways of modifying your text, pictures etc. However, RTF or wordpad format does not provide you such options, & even the options which are there are limited.
Above all, if you copy paste, or do a save as to your files, it will give you an error, as the compatibility will not be matched & the file will not have the same or correct data.
Still, you can save the text formats, but for images or high end data, it would be difficult.