Nasosoft.Documents.Transform supports converting HTML to RTF file. You can easily load a HTML document and transfer it into RTF file format in C# without understanding the RTF format details. For example:

[C#]
           //create a HtmlToRtfTransform
            using (HtmlToRtfTransform c = new HtmlToRtfTransform())
            {
                 c.Load("SimpleTest.htm"); //load the html file
                 c.Transform("SimpleTest.rtf"));//transform to rtf file
            }