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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (WordsDocument doc = Document.Load<WordsDocument>(@"D:\test.docx"))
{
    TextWatermark watermark = new TextWatermark("Protected image", new Font("Arial", 8));
    watermark.HorizontalAlignment = HorizontalAlignment.Center;
    watermark.VerticalAlignment = VerticalAlignment.Center;
    watermark.RotateAngle = 45;
    watermark.SizingType = SizingType.ScaleToParentDimensions;
    watermark.ScaleFactor = 1;

    foreach (WordsSection section in doc.Sections)
    {
        foreach (WordsShape shape in section.Shapes)
        {
            // Headers&Footers usually contains only service information.
            // So, we skip images in headers/footers, expecting that they are probably watermarks or backgrounds
            if (shape.HeaderFooter == null &&
               shape.Image != null)
            {
                shape.Image.AddWatermark(watermark);
            }
        }
    }
}