To reproduce: please refer to the attached sample project.

Workaround: export RadRichTextEditor's content to a .doc file. Then, use the RadWordsProcessing library to import the .doc file and export it as a pdf:

Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider provider = 
    new Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
string fileName = @"..\..\exported.doc";
string pdfFileName = @"..\..\exported.pdf";
Stream s = new FileStream(fileName,FileMode.Create, FileAccess.Write);
provider.Export(document, s);
s.Close();
s.Dispose();

Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider provider2 = 
    new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
using (Stream input = File.OpenRead(fileName))
{
    Telerik.Windows.Documents.Flow.Model.RadFlowDocument document2 = provider2.Import(input);
    Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider provider3 = 
        new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();
    using (Stream output = File.OpenWrite(pdfFileName))
    { 
        provider3.Export(document2, output);
    }
}

System.Diagnostics.Process.Start(pdfFileName);