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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)))
{
    TextWatermark textWatermark = new TextWatermark("This is a print only test watermark. It won't appear in view mode.", new Font("Arial", 8));
    bool isPrintOnly = true;

    // Annotation will be printed, but not displayed in pdf viewing application
    doc.Pages[0].AddAnnotationWatermark(textWatermark, isPrintOnly);

    doc.Save();
}