Converting Blob Data (PDF) from SQL Database to a PDF-File

In my Datebase Table the PDFs are saved as Blob Data, example:

What I'm trying to do now is to create a PDF file out of this data.

My code is like that:

SqlConnection con = new SqlConnection(connectionString);
        con.Open();
        if (con.State == ConnectionState.Open)
        { 
            string query = // fancy SELECTION string goes here... reads only one by the way

            using (SqlCommand command = new SqlCommand(query, con))
            {
                using (SqlDataReader reader = command.ExecuteReader())
                {
                    while (reader.Read())
                    {
                        Byte[] bytes = (Byte[])reader["File BLOB-Contents"];
                        Console.WriteLine(bytes.Length); // prints the correct file size in Bytes
                        using (FileStream fstream = new FileStream(@"C:\Users\myUsername\Desktop\test3.pdf", FileMode.OpenOrCreate, FileAccess.ReadWrite))
                        {
                            fstream.Write(bytes, 0, bytes.Length);
                        }
                    }
                }
            }
        }

The pdf gets created in the end but the problem is, that I can't open it. I get the following (German) message in Adobe Reader:

Anyone here an idea or is there something I'm doing wrong? The file size is ok. It's not 0.