I found it! The file to be edited is:
\classes\Search.php
The lines from linenumber289 should look like this only 2rows and 1 attribute to the query have to be added :
$weightArray = array(
'pname' => Configuration::get('PS_SEARCH_WEIGHT_PNAME'),
'reference' => Configuration::get('PS_SEARCH_WEIGHT_REF'),
'pareference' => Configuration::get('PS_SEARCH_WEIGHT_REF'), // add just this line here
'ean13' => Configuration::get('PS_SEARCH_WEIGHT_REF'),
'description_short' => Configuration::get('PS_SEARCH_WEIGHT_SHORTDESC'),
'description' => Configuration::get('PS_SEARCH_WEIGHT_DESC'),
'cname' => Configuration::get('PS_SEARCH_WEIGHT_CNAME'),
'mname' => Configuration::get('PS_SEARCH_WEIGHT_MNAME'),
'tags' => Configuration::get('PS_SEARCH_WEIGHT_TAG'),
'attributes' => Configuration::get('PS_SEARCH_WEIGHT_ATTRIBUTE'),
'features' => Configuration::get('PS_SEARCH_WEIGHT_FEATURE')
);
$products = $db->ExecuteS('
SELECT p.id_product, pl.id_lang, pl.name as pname, p.reference, p.ean13, pl.description_short, pl.description, cl.name as cname, m.name as mname,
pa.reference as pareference
FROM '._DB_PREFIX_.'product p
LEFT JOIN '._DB_PREFIX_.'product_lang pl ON p.id_product = pl.id_product
LEFT JOIN '._DB_PREFIX_.'category_lang cl ON (cl.id_category = p.id_category_default AND pl.id_lang = cl.id_lang)
LEFT JOIN '._DB_PREFIX_.'manufacturer m ON m.id_manufacturer = p.id_manufacturer
left join '._DB_PREFIX_.'product_attribute pa ON pa.id_product = p.id_product
WHERE p.indexed = 0', false); // add just this line above this comment
Than you have to go to your backend admin>preferences>search> and run complete indexation of all products.