WooCommerce product attributes are actually custom fields. On the General Options -> Sources panel select the ones which begin with the prefix “attribute_”: http://i.imgur.com/v2CtY6h.png
“I already know that is possible filter by attrbutes, but what I want is to search not just by titles but also by product specifications.”
I thought you meant searching the product attributes content, not using options on front-end.
If you want to make a dropdown/checkbox/radio etc.. visible filters for product attributes, then you need to create custom field filters: https://goo.gl/6dKjaP