using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;

 static void Main(string[] args)
        {
            string strDoc = "Sample.doc";
            string strTxt = "Append text in body - OpenAndAddTextToWordDocument";
            OpenAndAddTextToWordDocument(strDoc, strTxt);
        }

        private static void OpenAndAddTextToWordDocument(string filepath, string txt)
        {
            // Open a WordprocessingDocument for editing using the filepath.
            WordprocessingDocument wordprocessingDocument =
                WordprocessingDocument.Open(filepath, true);

            // Assign a reference to the existing document body.
            Body body = wordprocessingDocument.MainDocumentPart.Document.Body;

            // Add new text.
            Paragraph para = body.AppendChild(new Paragraph());
            Run run = para.AppendChild(new Run());
            run.AppendChild(new Text(txt));

            // Close the handle explicitly.
            wordprocessingDocument.Close();
        }
    }

 using Aspose.Words
 Document doc = new Document("Sample Aspose.doc");

            DocumentBuilder builder = new DocumentBuilder(doc);
            builder.Write("Insert text");
            doc.Save("Add text Out.doc");