Is there any extension that can help me automatically attache PDF invoices to email to customers and also manage them systematically?
@mage_goose I am not sure what you mean by managing systematically but I offer Email Attachments https://marketplace.magento.com/catalogsearch/result/?q=email%20attachments which allows you to attach the invoice pdf to the invoice confirmation email - both for Magento 1 and 2.
