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

namespace EncryptPublicKey
{
    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));

                var readPassword = Encoding.UTF8.GetBytes("123");//it can be null.
                var editPassword = Encoding.UTF8.GetBytes("456");
                int permissions = PdfWriter.ALLOW_PRINTING | PdfWriter.ALLOW_COPY;
                pdfWriter.SetEncryption(readPassword, editPassword, permissions, PdfWriter.STRENGTH128BITS);

                pdfDoc.Open();

                pdfDoc.Add(new Phrase("tst 0"));
                pdfDoc.NewPage();
                pdfDoc.Add(new Phrase("tst 1"));
            }

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

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.X509;

namespace EncryptPublicKey
{
    /// <summary>
    /// A Personal Information Exchange File Info
    /// </summary>
    public class PfxData
    {
        /// <summary>
        /// Represents an X509 certificate
        /// </summary>
        public X509Certificate[] X509PrivateKeys { set; get; }

        /// <summary>
        /// Certificate's public key
        /// </summary>
        public ICipherParameters PublicKey { set; get; }
    }
}

using System;
using System.IO;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Pkcs;
using Org.BouncyCastle.X509;

namespace EncryptPublicKey
{
    /// <summary>
    /// A Personal Information Exchange File Reader
    /// </summary>
    public class PfxReader
    {
        X509Certificate[] _chain;
        AsymmetricKeyParameter _asymmetricKeyParameter;
       
        /// <summary>
        /// Reads A Personal Information Exchange File.
        /// </summary>
        /// <param name="pfxPath">Certificate file's path</param>
        /// <param name="pfxPassword">Certificate file's password</param>
        public PfxData ReadCertificate(string pfxPath, string pfxPassword)
        {
            using (var stream = new FileStream(pfxPath, FileMode.Open, FileAccess.Read))
            {
                var pkcs12Store = new Pkcs12Store(stream, pfxPassword.ToCharArray());
                var alias = findThePublicKey(pkcs12Store);
                _asymmetricKeyParameter = pkcs12Store.GetKey(alias).Key;
                constructChain(pkcs12Store, alias);
                return new PfxData { X509PrivateKeys = _chain, PublicKey = _asymmetricKeyParameter };
            }
        }

        private void constructChain(Pkcs12Store pkcs12Store, string alias)
        {
            var certificateChains = pkcs12Store.GetCertificateChain(alias);
            _chain = new X509Certificate[certificateChains.Length];

            for (int k = 0; k < certificateChains.Length; ++k)
                _chain[k] = certificateChains[k].Certificate;
        }

        private static string findThePublicKey(Pkcs12Store pkcs12Store)
        {
            string alias = string.Empty;
            foreach (string entry in pkcs12Store.Aliases)
            {
                if (pkcs12Store.IsKeyEntry(entry) && pkcs12Store.GetKey(entry).Key.IsPrivate)
                {
                    alias = entry;
                    break;
                }
            }

            if (string.IsNullOrEmpty(alias))
                throw new NullReferenceException("Provided certificate is invalid.");

            return alias;
        }
    }
}

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

namespace EncryptPublicKey
{
    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));

                var certs = new PfxReader().ReadCertificate(@"D:\path\cert.pfx", "123");
                pdfWriter.SetEncryption(
                          certs: certs.X509PrivateKeys,
                          permissions: new int[] { PdfWriter.ALLOW_PRINTING, PdfWriter.ALLOW_COPY },
                          encryptionType: PdfWriter.ENCRYPTION_AES_128);

                pdfDoc.Open();

                pdfDoc.Add(new Phrase("tst 0"));
                pdfDoc.NewPage();
                pdfDoc.Add(new Phrase("tst 1"));
            }

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

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

namespace EncryptPublicKey
{
    class Program
    {
        static void Main(string[] args)
        {
            PdfReader reader = new PdfReader("TestDec.pdf");
            using (var stamper = new PdfStamper(reader, new FileStream("TestEnc.pdf", FileMode.Create)))
            {
                var certs = new PfxReader().ReadCertificate(@"D:\path\cert.pfx", "123");
                stamper.SetEncryption(
                         certs: certs.X509PrivateKeys,
                         permissions: new int[] { PdfWriter.ALLOW_PRINTING, PdfWriter.ALLOW_COPY },
                         encryptionType: PdfWriter.ENCRYPTION_AES_128);
                stamper.Close();
            }

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