using System.Diagnostics;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

namespace PDFAttachment
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var pdfDoc = new Document(PageSize.A4))
            {
                var pdfWriter = PdfWriter.GetInstance(pdfDoc, new FileStream("Test.pdf", FileMode.Create));                
                pdfDoc.Open();

                pdfDoc.Add(new Phrase("Test"));

                var fs = PdfFileSpecification.FileEmbedded(pdfWriter, @"C:\path\logo.png", "logo.png", null);
                pdfWriter.AddFileAttachment("توضیحات",fs);
            }

            Process.Start("Test.pdf");
        }
    }
}

using System.Diagnostics;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

namespace PDFAttachment
{
    class Program
    {
        static void Main(string[] args)
        {
            using (var pdfDoc = new Document(PageSize.A4))
            {
                var pdfWriter = PdfWriter.GetInstance(pdfDoc, new FileStream("Test.pdf", FileMode.Create));                
                pdfDoc.Open();

                pdfDoc.Add(new Phrase("Test"));

                var filePath = @"C:\path\logo.png";
                var fileInfo = new FileInfo(filePath);
                var pdfDictionary = new PdfDictionary();
                pdfDictionary.Put(PdfName.MODDATE, new PdfDate(fileInfo.LastWriteTime));
                var fs = PdfFileSpecification.FileEmbedded(pdfWriter, filePath, fileInfo.Name, null, true, null, pdfDictionary);
                pdfWriter.AddFileAttachment("توضیحات", fs);
            }

            Process.Start("Test.pdf");
        }
    }
}

pdfWriter.ViewerPreferences = PdfWriter.PageModeUseAttachments;

using System.Diagnostics;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

namespace PDFAttachment
{
    class Program
    {
        static void Main(string[] args)
        {
            var reader = new PdfReader("Test.pdf");
            using (var stamper = new PdfStamper(reader, new FileStream("newTest.pdf", FileMode.Create)))
            {
                var filePath = @"C:\path\logo.png";
                addAttachment(stamper, filePath, "توضیحات");
                stamper.Close();
            }

            Process.Start("newTest.pdf");
        }

        private static void addAttachment(PdfStamper stamper, string filePath, string description)
        {
            var fileInfo = new FileInfo(filePath);
            var pdfDictionary = new PdfDictionary();
            pdfDictionary.Put(PdfName.MODDATE, new PdfDate(fileInfo.LastWriteTime));
            var pdfWriter = stamper.Writer;
            var fs = PdfFileSpecification.FileEmbedded(pdfWriter, filePath, fileInfo.Name, null, true, null, pdfDictionary);
            stamper.AddFileAttachment(description, fs);
        }
    }
}