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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)))
{
    // Initialize image or text watermark
    TextWatermark watermark = new TextWatermark("Do not copy", new Font("Arial", 8));
    watermark.HorizontalAlignment = HorizontalAlignment.Center;
    watermark.VerticalAlignment = VerticalAlignment.Center;
    watermark.RotateAngle = 45;
    watermark.SizingType = SizingType.ScaleToParentDimensions;
    watermark.ScaleFactor = 1;
    watermark.Opacity = 0.5;

    // Add watermark of any type first
    doc.Pages[0].AddWatermark(watermark);

    // Rasterize the first page
    doc.Pages[0].Rasterize(100, 100, PdfImageConversionFormat.Png);

    // Content of the first page is replaced with raster image
    doc.Save();
}