Word.Application newApp = new Word.Application(); 

                // specifying the Source & Target file names
                object Source="c:\\abc\\Source.doc";
                object Target="c:\\abc\\Target.rtf";

                // Use for the parameter whose type are not known or  
                // say Missing
                object Unknown =Type.Missing;

                // Source document open here
                // Additional Parameters are not known so that are  
                // set as a missing type
                newApp.Documents.Open(ref Source,ref Unknown, 
                     ref Unknown,ref Unknown,ref Unknown, 
                     ref Unknown,ref Unknown,ref Unknown, 
                     ref Unknown,ref Unknown,ref Unknown, 
                     ref Unknown,ref Unknown,ref Unknown,ref Unknown);

        // Specifying the format in which you want the output file 
                object format = Word.WdSaveFormat.wdFormatRTF;

        //Changing the format of the document
                newApp.ActiveDocument.SaveAs(ref Target,ref format, 
                        ref Unknown,ref Unknown,ref Unknown, 
                        ref Unknown,ref Unknown,ref Unknown, 
                        ref Unknown,ref Unknown,ref Unknown, 
                        ref Unknown,ref Unknown,ref Unknown, 
                        ref Unknown,ref Unknown);                    

                // for closing the application
                newApp.Quit(ref Unknown,ref Unknown,ref Unknown);