using (var doc = WordprocessingDocument.Open("myfile.docx", true)
{
    foreach (var par in doc.MainDocumentPart.RootElement.Descendants<Paragraph>())
    {
        foreach (var run in par.Elements<Run>())
        {
            if (run.GetFirstChild<SymbolChar>().Char.HasValue)
                richTextBox1.Text += run.GetFirstChild<SymbolChar>().Char.Value
        }
    }
}