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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (Stream watermarkStream = File.OpenRead(@"D:\watermark.jpg"))
{
    using (Document doc = Document.Load(Utilities.MapSourceFilePath (PngFilePath)))
    {
        // Use stream containing an image as constructor parameter
        using (ImageWatermark watermark = new ImageWatermark(watermarkStream))
        {
            // Add watermark to the document
            doc.AddWatermark(watermark);

            doc.Save();
        }
    }
}