Add border to PDF Image using iTextSharp in ASP.Net

You need to set Image Border to Rectangle.BOX and BorderWidth property before adding to PDFCell.

Check with below code.

PdfPCell val21 = new PdfPCell(new Phrase(""));
val21.BorderColor = (BaseColor.RED);
val21.Border=(5);
val21.Colspan=(3);
val21.PaddingTop=(60f);
val21.PaddingLeft=(25f);
PdfPCell pdfPCell3 = val21;
pdfPCell3.AddElement((IElement)new Phrase(Environment.NewLine));
string attribute = ((BaseEntity)(object)shipment).GetAttribute<string>("ReturnBarcodePng");
if (!string.IsNullOrEmpty(attribute))
{
    using (MemoryStream stream = new MemoryStream(Convert.FromBase64String(attribute)))
    {
        
        System.Drawing.Image image = System.Drawing.Image.FromStream(stream);
        image.RotateFlip(System.Drawing.RotateFlipType.Rotate90FlipNone);
     
        Image ınstance = Image.GetInstance(image, System.Drawing.Imaging.ImageFormat.Png);
        ınstance.SetDpi(200, 200);
        ınstance.SetAbsolutePosition(0f, 0f);
        ınstance.ScaleToFit(450f, 420f);
        ınstance.Border = Rectangle.BOX;
        ınstance.BorderColor = Color.Black;
        ınstance.BorderWidth = 5f;
 
        pdfPCell3.AddElement((IElement)(object)ınstance);
 
        pdfPTable3.AddCell(pdfPCell3);
    }
}