convertandaddwatermark.cs

using (Converter converter = new Converter("sample.docx"))
{
    WatermarkOptions watermark = new WatermarkOptions
    {
        Text = "Sample watermark",
        Color = Color.Red,
        Width = 100,
        Height = 100,
        Background = true
    };
    PdfConvertOptions options = new PdfConvertOptions
    {
        Watermark = watermark
    };
    converter.Convert("converted.pdf", options);
}