PM> Install-Package Spire.PDF

using System.Drawing;
using Spire.Pdf;
using Spire.Pdf.Graphics;
using Spire.Pdf.Annotations;
using Spire.Pdf.Annotations.Appearance;

namespace AddDynamicStamp
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a PdfDocument object
            PdfDocument doc = new PdfDocument();

            // Load a PDF document
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\input.pdf");

            // Get a specific page
            PdfPageBase page = doc.Pages[1];

            // Create a PdfTemplate object
            PdfTemplate template = new PdfTemplate(220, 50);

            // Create two fonts
            PdfTrueTypeFont font1 = new PdfTrueTypeFont(new Font("Elephant", 16f, FontStyle.Bold),true);  
            PdfTrueTypeFont font2 = new PdfTrueTypeFont(new Font("Times New Roman", 10f, FontStyle.Bold),true);

            // Create a solid brush and a gradient brush
            PdfSolidBrush solidBrush = new PdfSolidBrush(Color.Blue);
            RectangleF rectangle1 = new RectangleF(new PointF(0, 0), template.Size);
            PdfLinearGradientBrush linearGradientBrush = new PdfLinearGradientBrush(rectangle1, new PdfRGBColor(Color.White), new PdfRGBColor(Color.Blue), PdfLinearGradientMode.Horizontal);

            // Create a pen
            PdfPen pen = new PdfPen(solidBrush);

            // Create a rounded rectangle path
            int CornerRadius = 10;
            PdfPath path = new PdfPath();
            path.AddArc(template.GetBounds().X, template.GetBounds().Y, CornerRadius, CornerRadius, 180, 90);
            path.AddArc(template.GetBounds().X + template.Width - CornerRadius, template.GetBounds().Y, CornerRadius, CornerRadius, 270, 90);
            path.AddArc(template.GetBounds().X + template.Width - CornerRadius, template.GetBounds().Y + template.Height - CornerRadius, CornerRadius, CornerRadius, 0, 90);
            path.AddArc(template.GetBounds().X, template.GetBounds().Y + template.Height - CornerRadius, CornerRadius, CornerRadius, 90, 90);
            path.AddLine(template.GetBounds().X, template.GetBounds().Y + template.Height - CornerRadius, template.GetBounds().X, template.GetBounds().Y + CornerRadius / 2);

            // Draw path on the template
            template.Graphics.DrawPath(pen, path);
            template.Graphics.DrawPath(linearGradientBrush, path);

            // Draw text on the template
            String string1 = "APPROVED\n";
            String string2 = "By Marketing Manager at " + DateTime.Now.ToString("HH:mm, MMM dd, yyyy");
            template.Graphics.DrawString(string1, font1, solidBrush, new PointF(5, 5));
            template.Graphics.DrawString(string2, font2, solidBrush, new PointF(2, 28));

            // Create a rubber stamp, specifying its size and location
            RectangleF rectangle2 = new RectangleF(55, page.ActualSize.Height - 55 - 70, 240, 55);
            PdfRubberStampAnnotation stamp = new PdfRubberStampAnnotation(rectangle2);

            // Create a PdfAppearance object and apply the template as its normal state
            PdfAppearance apprearance = new PdfAppearance(stamp);
            apprearance.Normal = template;

            // Apply the appearance to stamp
            stamp.Appearance = apprearance;

            // Add the stamp annotation to annotation collection
            page.Annotations.Add(stamp);

            // Save the file
            doc.SaveToFile("DynamicStamp.pdf", FileFormat.PDF);

            // Dispose resources
            doc.Dispose();
        }
    }
}

using Spire.Pdf;
using Spire.Pdf.Annotations;
using Spire.Pdf.Annotations.Appearance;
using Spire.Pdf.Graphics;
using System.Drawing;

namespace AddImageStamp
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a PdfDocument object
            PdfDocument doc = new PdfDocument();

            // Load a PDF document
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\input.pdf");

            // Get a specific page
            PdfPageBase page = doc.Pages[1];

            // Load an image file
            PdfImage image = PdfImage.FromFile("C:\\Users\\Administrator\\Desktop\\confidential.png");

            // Get the width and height of the image
            int width = image.Width;
            int height = image.Height;

            // Create a PdfTemplate object based on the size of the image
            PdfTemplate template = new PdfTemplate(width, height, true);

            // Draw image on the template
            template.Graphics.DrawImage(image, 0, 0, width, height);

            // Create a rubber stamp annotation, specifying its location and position
            RectangleF rect = new RectangleF(page.ActualSize.Width - width - 50, page.ActualSize.Height - height - 50, width, height);
            PdfRubberStampAnnotation stamp = new PdfRubberStampAnnotation(rect);

            // Create a PdfAppearance object
            PdfAppearance pdfAppearance = new PdfAppearance(stamp);

            // Set the template as the normal state of the appearance
            pdfAppearance.Normal = template;

            // Apply the appearance to the stamp
            stamp.Appearance = pdfAppearance;

            // Add the stamp annotation to PDF
            page.Annotations.Add(stamp);

            // Save the file
            doc.SaveToFile("ImageStamp.pdf");

            // Dispose resources
            doc.Dispose();
        }
    }
}