public string AppendPdfList(string[] append)
{
           PdfFormatProvider provider = new PdfFormatProvider();
            RadFixedDocument targetDocument = new RadFixedDocument();
            provider.ImportSettings.DocumentUnhandledException += (o, args) => { args.Handled = true; };

            // Iterate through the files you would like to merge 
            foreach (string documentName in append)
            {
               RadFixedDocument sourceDocument = provider.Import(File.ReadAllBytes(documentName));
               targetDocument.Merge(sourceDocument);
            }
            TelerikPdfHelper.ClearDuplicatedFonts(targetDocument);

            OutputFilename = Path.GetTempFileName();
           File.WriteAllBytes(OutputFilename, provider.Export(targetDocument));
            return OutputFilename;
}

Results in System.NotImplementedException: 'The method or operation is not implemented.