Length of the data to decrypt is invalid

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C#

buff    = new byte[1600];//decryption buffer.
                byteKey = GetByteKeys256(strKey);

                outputStream     = new MemoryStream();
                objSymmAlgorithm = new Cryptography.RijndaelManaged();

                decStream = new Cryptography.CryptoStream(outputStream, objSymmAlgorithm.CreateDecryptor(byteKey, VECTOR_IV_256), Cryptography.CryptoStreamMode.Write);
                //do the deryption ...
                inputStream.Position = 0;
                while( (lenRead = inputStream.Read(buff, 0,buff.Length))>0)
                {
                    decStream.Write(buff, 0, lenRead);
                }
                decStream.FlushFinalBlock();                outputStream.Flush();
                return outputStream;

it will give me error as Length of the data to decrypt is invalid while decrypting pdf file only for some file while decrypting for other pdf file it work fine.what to do to avoid this any one have idea?