Different way to add watermarks in PDF | https://docs.conholdate.com/net/watermarks-in-pdf/

PdfAddAnnotationWatermark.cs

PdfLoadOptions loadOptions = new PdfLoadOptions();
// Constants.InDocumentPdf is an absolute or relative path to your document. Ex: @"C:\Docs\document.pdf"
using (Watermarker watermarker = new Watermarker(Constants.InDocumentPdf, loadOptions))
{
    PdfAnnotationWatermarkOptions options = new PdfAnnotationWatermarkOptions();
    // Add text watermark
    TextWatermark textWatermark = new TextWatermark("This is a annotation watermark", new Font("Arial", 8));
    textWatermark.HorizontalAlignment = HorizontalAlignment.Left;
    textWatermark.VerticalAlignment = VerticalAlignment.Top;
    watermarker.Add(textWatermark, options);
    // Add image watermark
    using (ImageWatermark imageWatermark = new ImageWatermark(Constants.ProtectJpg))
    {
        imageWatermark.HorizontalAlignment = HorizontalAlignment.Right;
        imageWatermark.VerticalAlignment = VerticalAlignment.Top;
        watermarker.Add(imageWatermark, options);
    }

    watermarker.Save(Constants.OutDocumentPdf);
}

PdfAddArtifactWatermark.cs

PdfLoadOptions loadOptions = new PdfLoadOptions();
// Constants.InDocumentPdf is an absolute or relative path to your document. Ex: @"C:\Docs\document.pdf"
using (Watermarker watermarker = new Watermarker(Constants.InDocumentPdf, loadOptions))
{
    PdfArtifactWatermarkOptions options = new PdfArtifactWatermarkOptions();
    // Add text watermark
    TextWatermark textWatermark = new TextWatermark("This is an artifact watermark", new Font("Arial", 8));
    textWatermark.HorizontalAlignment = HorizontalAlignment.Right;
    watermarker.Add(textWatermark, options);
    // Add image watermark
    using (ImageWatermark imageWatermark = new ImageWatermark(Constants.LogoBmp))
    {
        watermarker.Add(imageWatermark, options);
    }
    watermarker.Save(Constants.OutDocumentPdf);
}

PdfAddPrintOnlyAnnotationWatermark.cs

PdfLoadOptions loadOptions = new PdfLoadOptions();
// Constants.InDocumentPdf is an absolute or relative path to your document. Ex: @"C:\Docs\document.pdf"
using (Watermarker watermarker = new Watermarker(Constants.InDocumentPdf, loadOptions))
{
    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
    PdfAnnotationWatermarkOptions options = new PdfAnnotationWatermarkOptions();
    options.PageIndex = 0;
    options.PrintOnly = isPrintOnly;

    watermarker.Add(textWatermark, options);
    watermarker.Save(Constants.OutDocumentPdf);
}

Read the complete article: https://docs.conholdate.com/net/watermarks-in-pdf/