PdfXObjectWatermarkOptions

public sealed class PdfXObjectWatermarkOptions : PdfWatermarkOptions

PdfLoadOptions loadOptions = new PdfLoadOptions();
using (Watermarker watermarker = new Watermarker(@"C:\doc.pdf", loadOptions))
using (ImageWatermark watermark = new ImageWatermark(@"C:\watermark.png"))
{
    PdfXObjectWatermarkOptions options = new PdfXObjectWatermarkOptions();
    options.PageIndex = 0;

    watermarker.Add(watermark, options);
    watermarker.Save();
}