I don't understand. I've open many versions of Projeqtor and this file is not present. Where is the problem with my config ?

I confim that this file does not exist.
And PDF files are correctly generated without it :silly:
(You can check at https://demo.projeqtor.org)

Could you post exact message logged ?
Which version of PHP are you using ?

(edit) I found issues about using html2pdf with PHP =5.2

I use PHP 5.5.

Here is the log trace :

2015-12-02 18:47:39.279 ===== TRACE ===== Cron started at 02/12/2015 18:47:39
2015-12-02 18:48:49.700 ** ERROR ** Impossible to load class HTML2PDF_locale
=> Not found in ../model/custom/HTML2PDF_locale.php
=> Not found in ../model/HTML2PDF_locale.php
=> Not found in ../model/persistence/HTML2PDF_locale.php

2015-12-02 18:49:00.989 ** ERROR ** Impossible to load class HTML2PDF_locale
=> Not found in ../model/custom/HTML2PDF_locale.php
=> Not found in ../model/HTML2PDF_locale.php
=> Not found in ../model/persistence/HTML2PDF_locale.php

Thanks for your help

It is a issue produced with PHP < 5.3
To have this work for PHP < 5.3, comment lines 4005 to 4007 in /model/persistence/SqlElement.php from

//	  if (! is_a($className, 'SqlElement', true )) {
//	    traceHack("Class '$className' does not extend SqlElement");
//	  }

Note that this mau introduce secuirity leak.
Better solution will be proposed in V5.2.

Very stange, it seems that html2pdf class used in your case is not the one included in PRojeQtOr.
The expected correct file /external/html2pdf/html2pdf.class.php starts with :

 PDF convertor
 * distributed under the LGPL License
 *
 * @author  Laurent MINGUET 
 * @version 4.03
 */
 
if (!defined('__CLASS_HTML2PDF__')) {

    define('__CLASS_HTML2PDF__', '4.03');
    define('HTML2PDF_USED_TCPDF_VERSION', '5.0.002');
	
    require_once(dirname(__FILE__).'/_class/exception.class.php');
    require_once(dirname(__FILE__).'/_class/locale.class.php');
    require_once(dirname(__FILE__).'/_class/myPdf.class.php');
    require_once(dirname(__FILE__).'/_class/parsingHtml.class.php');
    require_once(dirname(__FILE__).'/_class/parsingCss.class.php');

    class HTML2PDF

The line
require_once(dirname(__FILE__).'/_class/locale.class.php');
includes correct class file for HTML2PDF_locale objects, so ProjeQtOr autoloader should not try to load this class definition.

1) What is your PHP version
2) what is your PHP stack (zend, linux php, WAMP, ...)
3) do you have other PHP applications running on same server, and if yes do they also usde HTML2PDF ?

I've reinstalled projeqtor on an other server and now all is fine !

Thanks