How to create single PDF from multiple rtf files

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C#

Windows

application

Actually i have multiple rtf files.I want to create single PDF from this multiple file.actually i am trying this but only single rtf file convert into pdf using SpireDoc.
this is code for pdf from single PDF.....

private void button1_Click(object sender, EventArgs e)
       {
           //Create word document
           Document document = new Document();

 Section section = document.AddSection();            
            document.LoadFromFile("KPI000006.rtf", FileFormat.Rtf);                      
            //document.SaveToFile("New.pdf", FileFormat.PDF);
            document.Sections[0].Paragraphs[3].AppendBreak(BreakType.PageBreak);
            document.LoadFromFile("KPI000007.rtf", FileFormat.Rtf);//Launching the pdf reader to open.
            document.SaveToFile("New.pdf", FileFormat.PDF);
            FileViewer("New.pdf");
        }
    private void FileViewer(string fileName)
        {
            try
            {
                System.Diagnostics.Process.Start(fileName);
            }
            catch { }
        }

so please give me code for PDF file from multiple rtf files...