GetContent

public T GetContent<T>()
    where T : Content

PdfLoadOptions loadOptions = new PdfLoadOptions();
using (Watermarker watermarker = new Watermarker(@"C:\doc.pdf", loadOptions))
{ 
    using (ImageWatermark watermark = new ImageWatermark(@"C:\watermark.png"))
    {
        watermarker.Add(watermark);
    }

    PdfContent content = watermarker.GetContent<PdfContent>();
    content.Rasterize(300, 300, PdfImageConversionFormat.Png);

    watermarker.Save();
}