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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (Document doc = Document.Load(Utilities.MapSourceFilePath (PngFilePath)))
{
    Font font = new Font("Calibri", 12);
    TextWatermark watermark = new TextWatermark("This is a test watermark", font);

    // Set sizing type
    watermark.SizingType = SizingType.ScaleToParentDimensions;

    // Set watermark scale
    watermark.ScaleFactor = 0.5;

    doc.AddWatermark(watermark);
    doc.Save();
}