In this article I will explain with an example, how to add Watermark to existing PDF using iTextSharp in ASP.Net Core (.Net Core) MVC.
Note: For beginners in ASP.Net Core (.Net Core 7) MVC, please refer my article ASP.Net Core 7: Hello World Tutorial with Sample Program example.
 
 

Download iTextSharp and XmlWorkerHelper Libraries

You can download the iTextSharp and XmlWorkerHelper libraries from the following links.
iTextSharp
XmlWorkerHelper
Note: You will need to add the reference of iTextSharp and XmlWorkerHelper libraries in your project.
 
 

PDF File and Watermark image Location

The sample PDF file and Watermark image are located inside the PDF and Images Folder (Directory) of wwwroot Folder (Directory).
ASP.Net Core: Add Watermark to existing PDF using iTextSharp
 
 

Namespaces

You will need to import the following namespace.
using iTextSharp.text.pdf;
 
 

Controller

Inside the Controller, first the private property IWebHostEnvironment interface is created.
Then, the interface is injected into the Constructor (HomeController) using Dependency Injection method and the injected object is assigned to private property (created earlier).
The Controller consists of following Action method.

Action method for handling GET operation

Inside this Action method, simply the View is returned.
 

Action method for handling POST operation

Inside this Action method, first the BYTE Array of the sample PDF file (explained earlier) is determined using IWebHostEnvironment interface.
Note: For more details about IWebHostEnvironment interface, please refer my article Using IWebHostEnvironment in ASP.Net Core. For more details on how to access Static Files in Core, please refer my article Static Files (Images, CSS and JS files) in ASP.Net Core.
 
Then, the Watermark image is read using GetInstance method of Image class and its position is set using SetAbsolutePosition method of Image class.
After that, an object of PdfReader class is created which accepts BYTE Array of the sample PDF file as a parameter.
A PdfStamper class object is also created which accepts objects of PdfReader and MemoryStream class as a parameter.
Then, PDF document is created using existing PDF file and a FOR loop is executed over all the pages.
 

Adding Watermark

Inside the loop, the Watermark image is added to the PDF document using AddImage method of PdfContentByte class.
Finally, the File function is called which accepts the BYTE Array of the PDF document and the name of the file, which initiates the file download operation.
public class HomeController : Controller
{
    private IWebHostEnvironment  Environment { get; }
    public HomeController(IWebHostEnvironment _environment)
    {
        this.Environment = _environment;
    }
 
    public IActionResult Index()
    {
        return View();
    }
 
    [HttpPost]
    public IActionResult Export()
    {
        byte[] bytes = System.IO.File.ReadAllBytes(Path.Combine(this.Environment.WebRootPath,"PDF\\Sample.pdf"));
 
        // Reading watermark image.
        iTextSharp.text.Image img = iTextSharp.text.Image.GetInstance(Path.Combine(this.Environment.WebRootPath,"Images\\Watermark.png"));
 
        // Setting watermark image position.
        img.SetAbsolutePosition(100, 200);
 
        using (MemoryStream  stream =  new MemoryStream())
        {
            // Reading pdf.
            PdfReader  reader =  new PdfReader(bytes);
            using (PdfStamper  stamper =  new PdfStamper(reader, stream))
            {
                // Loop through all pages.
                for (int i = 1; i <= reader.NumberOfPages; i++)
                {
                    PdfContentByte pdfContentByte = stamper.GetUnderContent(i);
 
                    // Adding Image to the pdf.
                    pdfContentByte.AddImage(img);
                }
            }
 
            bytes = stream.ToArray();
            return  File(bytes,"application/pdf","Sample_With_Watermark.pdf");
        }
    }
}
 
 

View

HTML Markup

The View consists of an HTML Form which has been created using the following TagHelpers attributes.
asp-action – Name of the Action. In this case the name is Index.
asp-controller – Name of the Controller. In this case the name is Home.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
The Form consists of a Submit Button which when clicked, the Form is submitted.
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@{
     Layout =  null;
}
 
<!DOCTYPE html>
 
<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
</head>
<body>
    <form method="post" asp-action="Export" asp-controller="Home">
        <input id="btnSubmit" type="submit" value="Export" />
    </form>
</body>
</html>
 
 

Screenshots

PDF before Exporting

ASP.Net Core: Add Watermark to existing PDF using iTextSharp
 

Form

ASP.Net Core: Add Watermark to existing PDF using iTextSharp
 

PDF after Exporting

ASP.Net Core: Add Watermark to existing PDF using iTextSharp
 
 

Downloads