static void Main(string[] args)
        {
                      Telerik.Windows.Documents.Flow.Model.RadFlowDocument templateDocument = GetDocument("Template.rtf");
            Telerik.Windows.Documents.Flow.Model.RadFlowDocument contentDocument = GetDocument("Content.rtf");
            InsertDocumentOptions options = new InsertDocumentOptions();
            options.ConflictingStylesResolutionMode = ConflictingStylesResolutionMode.RenameSourceStyle;
            options.InsertLastParagraphMarker = true;

            RadFlowDocumentEditor editor = new RadFlowDocumentEditor(templateDocument);
            editor.InsertDocument(contentDocument, options);

            string mergedDocumentFilePath ="MergeDocumentsWithWordsProcessing.rtf";
            File.Delete(mergedDocumentFilePath);
            WriteDocToFile(templateDocument, mergedDocumentFilePath);

        }
        private static Telerik.Windows.Documents.Flow.Model.RadFlowDocument GetDocument(string rtfFilePath)
        {
            Telerik.Windows.Documents.Flow.Model.RadFlowDocument document = null;
            var rtfImporter = new Telerik.Windows.Documents.Flow.FormatProviders.Rtf.RtfFormatProvider();
            using (Stream stream = File.OpenRead(rtfFilePath))
            {
                document = rtfImporter.Import(stream);
            }
            return document;
        }


        private static void WriteDocToFile(Telerik.Windows.Documents.Flow.Model.RadFlowDocument doc, string filename)
        {
            var rtfExporter = new Telerik.Windows.Documents.Flow.FormatProviders.Rtf.RtfFormatProvider();
            string rtfText = rtfExporter.Export(doc);
            File.WriteAllText(filename, rtfText);

            Process.Start(filename);
        }

RadFlowDocument flowDocument;
using (Stream str = new FileStream("input.docx", FileMode.OpenOrCreate))
{
    MemoryStream ms = new MemoryStream();
    str.CopyTo(ms);
    ms.Seek(0, SeekOrigin.Begin);

    using (ZipArchive archive = ZipArchive.Update(ms, null))
    {
        var zipEntries = archive.Entries;

        // Skip glossary on importfor (int i = zipEntries.Count() - 1; i >= 0; i--)
        {
            var entry = zipEntries.ElementAt(i);
            string entryName = entry.FullName;
            if (Regex.IsMatch(entryName, @"\[trash\]"))
            {
                entry.Delete();
            }
        }
    }
}       

editor.ReplaceText(new Regex($"(?<=\\s|^|[\\c_]){placeholder}(?=\\s|$|[\\c_])"),"new content");