When you save a document in a word processing program such as Microsoft Word you have the ability to change the format from the standard .DOC or .DOCX and save in other formats such as .RTF. To do this click on File > Save as…

File Save image

Then, when the Save as… dialog window opens click on the drop down menu that says "Save as type:" and select Rich Text Format (*.rtf). 

Save As image

Enter the name of your document in the "File name:" box as you normally would and click the Save button.