PdfAnnotationWatermarkOptions

public sealed class PdfAnnotationWatermarkOptions : PdfWatermarkOptions

PdfLoadOptions loadOptions = new PdfLoadOptions();
using (Watermarker watermarker = new Watermarker(@"D:\test.pdf", loadOptions))
{
    using (ImageWatermark watermark = new ImageWatermark(@"D:\icon.png"))
    {
        PdfAnnotationWatermarkOptions options = new PdfAnnotationWatermarkOptions();
        options.PageIndex = -1; // डिफ़ॉल्ट - सभी पृष्ठ

        watermarker.Add(watermark, options);
    }

    watermarker.Save();
}