Rijndael not decrypting correctly

Question

We have an encrypted binary field in a SQL server DB with a key and we are trying to decrypt the file. It creates the PDF files that we need with the correct file names in the specified directory but when we try to open them we get an error that they were not decrypted correctly.

The file type is .PDF and we can tell by the size that there is content in the files.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.SqlClient;
using System.IO;
using System.Security.Cryptography;
using System.Configuration;

namespace PHDocExtract
{
    class Program
    {
        private static List<PHDocFile> GetPHDocFiles()
        {
            List<PHDocFile> docList = new List<PHDocFile>();
            PHDocFile docFile;
            Database db = new Database();
            SqlConnection connection = db.GetConnection();

            string sqlQuery = "select Filename, ENCRYPTION_KEY, Filedata from dbo.PHATTACH where RECORD_TYPE = 'client' and PHATTACH_KEY > 116 and PHATTACH_KEY < 170";

            SqlCommand command = new SqlCommand(sqlQuery, connection);
            SqlDataReader dataReader = command.ExecuteReader();

            while (dataReader.Read())
            {
                docFile = new PHDocFile();
                docFile.FileName = dataReader.GetString(0);                
                docFile.EncryptionKey = dataReader.GetString(1);
                docFile.FileData = dataReader.GetSqlBytes(2);
                docList.Add(docFile);
                docFile = null;
            }

            db.CloseConnection();

            return docList;
        }

        private static void DecryptFiles(List<PHDocFile> docFiles)
        {
            UnicodeEncoding unicodeEncoding = new UnicodeEncoding();
            MemoryStream returnStream = new MemoryStream();
            RijndaelManaged crypto = new RijndaelManaged();

            crypto.KeySize = 256;
            crypto.BlockSize = 128;
            crypto.Mode = CipherMode.CBC;

            foreach (PHDocFile doc in docFiles)
            {

                var key = new Rfc2898DeriveBytes(doc.EncryptionKey, unicodeEncoding.GetBytes(doc.EncryptionKey.Substring(0, 8)), 1000);
                crypto.Key = key.GetBytes(crypto.KeySize / 8);
                crypto.IV = key.GetBytes(crypto.BlockSize / 8);

                //fileStream.Seek(0, SeekOrigin.Begin);
                //Stream inputFileStream = fileStream;
                Stream inputFileStream = doc.FileData.Stream;
                CryptoStream cryptoStream = new CryptoStream(inputFileStream, crypto.CreateDecryptor(), CryptoStreamMode.Read);

                int data;
                try
                {
                    while ((data = cryptoStream.ReadByte()) != -1)
                    {
                        returnStream.WriteByte((byte)data);
                    }
                }
                catch (CryptographicException ex)
                {
                    // ¯\_(ツ)_/¯
                    Console.WriteLine(ex.ToString());
                }

                doc.DecryptedFileData = returnStream.ToArray();

                //if (cryptoStream != null)
                //    cryptoStream.Dispose();
                //if (inputFileStream != null)
                //    inputFileStream.Dispose();
            }
        }

        private static void WriteFiles(List<PHDocFile> docFiles)
        {
            string exportFilePath = ConfigurationManager.AppSettings["outputFolder"];
            FileStream fileStream;
            BinaryWriter binaryWriter;

            foreach (PHDocFile docFile in docFiles)
            {
                fileStream = new FileStream(exportFilePath + docFile.FileName, FileMode.Create, FileAccess.Write);
                binaryWriter = new BinaryWriter(fileStream);

                foreach (byte idx in docFile.DecryptedFileData)
                {
                    binaryWriter.Write(idx);
                    binaryWriter.Flush();
                }
                binaryWriter.Close();
                fileStream.Close();
            }
        }

        static void Main(string[] args)
        {
            List<PHDocFile> phDocFiles = new List<PHDocFile>();
            phDocFiles = GetPHDocFiles();
            DecryptFiles(phDocFiles);
            WriteFiles(phDocFiles);
        }
    }
}

Adobe Acrobat Reader could not open (file name) because it is wither not a supported file type or because the file has been damaged (for example, it was sent as an email attachment and wasn't correctly decoded).