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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (PdfDocument doc = Document.Load<PdfDocument>(Utilities.MapSourceFilePath(FilePath)))
{
    var watermark = new TextWatermark("Test watermark", new Font("Arial", 42));
    watermark.HorizontalAlignment = HorizontalAlignment.Right;
    watermark.VerticalAlignment = VerticalAlignment.Top;
    watermark.SizingType = SizingType.ScaleToParentDimensions;
    watermark.ScaleFactor = 1;

    doc.PageMarginType = PdfPageMarginType.BleedBox;
    watermark.ConsiderParentMargins = true;
    doc.AddWatermark(watermark);
    doc.Save();
}