RadDocument radDocument = null;
XamlFormatProvider xamlformatProvider = new XamlFormatProvider();
using (FileStream inputStream = new FileStream("input.xaml", FileMode.Open))
{
    Console.WriteLine("reading input file");
    radDocument = xamlformatProvider.Import(inputStream);
}

var provider = new Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
var bytes = provider.Export(radDocument);

var provider2 = new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
var flowDoc = provider2.Import(bytes);

var provider3 = new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();

using (Stream output = File.OpenWrite(@"output.pdf"))
{
    Console.WriteLine("writing output file");
    provider3.Export(flowDoc, output);
}

Console.WriteLine("done...");
Console.ReadKey();

RadDocument radDocument1 = RtfSourceProvider.Import(rtf1);
RadDocument radDocument2 = RtfSourceProvider.Import(rtf2);

AppendDocumentOptions options = new AppendDocumentOptions();
options.FirstSourceSectionPropertiesResolutionMode = SectionPropertiesResolutionMode.NoSectionBreak;

RadDocumentMerger merger = new RadDocumentMerger(radDocument1);
merger.AppendDocument(radDocument2, options);

RichText.Document = merger.Document;

 StyleManager.ApplicationTheme = new Office2016Theme();