public function testConvertStringToHexString()
 {
     $enc = new \Com\Tecnick\Pdf\Encrypt\Encrypt();
     $result = $enc->convertStringToHexString('');
     $this->assertEquals('', $result);
     $result = $enc->convertStringToHexString('hello world');
     $this->assertEquals('68656c6c6f20776f726c64', $result);
 }