public function testConvertHexStringToString()
 {
     $enc = new \Com\Tecnick\Pdf\Encrypt\Encrypt();
     $result = $enc->convertHexStringToString('');
     $this->assertEquals('', $result);
     $result = $enc->convertHexStringToString('68656c6c6f20776f726c64');
     $this->assertEquals('hello world', $result);
     $result = $enc->convertHexStringToString('68656c6c6f20776f726c642');
     $this->assertEquals('hello world ', $result);
 }