If your document store in database in binary format then
code for retrive the document is

string sqlQuery="write the select statement"
datareaderObject =cmd.executereader(sqlQuery,connectionname);
if(datareaderObject.read())
{
 Response.Buffer = false;
            Response.ClearHeaders();
            Response.ContentType = "application/octet-stream";
            Response.AddHeader("Content-Disposition",
"attachment; filename=" +
datareaderObject["docname"].ToString());

            //Code for streaming the object while writing
            const int ChunkSize = 1024;
            byte[] buffer = new byte[ChunkSize];
            byte[] binary = (datareaderObject["doc"]) as byte[];
            MemoryStream ms = new MemoryStream(binary);
            int SizeToWrite = ChunkSize;
            for (int i = 0; i < binary.GetUpperBound(0) - 1;
i = i + ChunkSize)
            {
                if (!Response.IsClientConnected) return;
                if (i + ChunkSize >= binary.Length)
SizeToWrite = binary.Length - i;
                byte[] chunk = new byte[SizeToWrite];
                ms.Read(chunk, 0, SizeToWrite);
                Response.BinaryWrite(chunk);
                Response.Flush();
            }
            Response.Close();
        }
    }

if document store in virtual path


 if (datareadear.Read())
        {
            FileInfo file = new
System.IO.FileInfo(Server.MapPath("datareadear["filename"].ToString());
            if (file.Exists)
            {
                Response.Clear();
                Response.AddHeader("content-disposition",
"attachment; filename=" + file.Name);
                Response.AddHeader("Content-Length",
file.Length.ToString());
                Response.ContentType =
"application/octet-stream";
                Response.WriteFile(file.FullName);
                Response.End();
            }
            else
            {
                Response.Redirect("../error.aspx?error=" +
"File dose not exits");
                Response.End();
            }
        }