Step 1 - Read the bytes of the file to encrypt:
 var fileBytes = File.ReadAllBytes(PathOfFileToEncrypt);

Step 2 - Call Method to Encrypt File
 var finalFile = EncryptFile(fileBytes, string.Format("{0}-Encrypted", Path.Combine(encryptionOutDirectory, cpiFileName)));

Here is the Method
        private static FileInfo EncryptFile(byte[] fileBytes, string outFileName)
        {
            string keyString = "KEY";

            byte[] keyBytes = ASCIIEncoding.UTF8.GetBytes(keyString);

            KeyParameter sk = ParameterUtilities.CreateKeyParameter("AES", keyBytes);
            RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider();
            byte[] iv = new byte[16];
            rngCsp.GetBytes(iv);

            var c = CipherUtilities.GetCipher("AES/CTR/NoPadding");

            c.Init(true, new ParametersWithIV(sk, iv));
            var encryptedData = c.DoFinal(fileBytes);

            string base64EncryptedOutputString = Convert.ToBase64String(encryptedData);

            string encryptedFileName = outFileName.Replace("-Encrypted", "_" + BitConverter.ToInt64(iv, 0).ToString());

            File.WriteAllText(encryptedFileName, base64EncryptedOutputString);

            return new FileInfo(encryptedFileName);
        }