Ladys and gentlemen, here is the solution:
$loader = new StandardAutoloader(array('autoregister_zf' => true));
$loader->registerNamespace('ZendPdf', APPPATH . "third_party/ZendPdf");
$loader->registerNamespace('ZendSearch', APPPATH . "third_party/ZendSearch");
$loader->register();
Life could be so easy ;)