Add

public void Add(Watermark watermark)

using (Watermarker watermarker = new Watermarker(@"D:\input.pdf"))
{
    TextWatermark textWatermark = new TextWatermark("DRAFT", new Font("Arial", 19));
    textWatermark.HorizontalAlignment = HorizontalAlignment.Center;
    textWatermark.VerticalAlignment = VerticalAlignment.Top;
    textWatermark.ConsiderParentMargins = true;
    textWatermark.ForegroundColor = Color.Red;
    textWatermark.IsBackground = true;
    textWatermark.Opacity = 0.5;
    watermarker.Add(textWatermark);

    using (ImageWatermark imageWatermark = new ImageWatermark(@"D:\draft.png"))
    {
        imageWatermark.HorizontalAlignment = HorizontalAlignment.Center;
        imageWatermark.VerticalAlignment = VerticalAlignment.Bottom;
        imageWatermark.ConsiderParentMargins = true;
        imageWatermark.IsBackground = true;
        imageWatermark.Opacity = 0.5;
        watermarker.Add(imageWatermark);
    }

    watermarker.Save(@"D:\output.pdf");
}

public void Add(Watermark watermark, WatermarkOptions options)

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();
}