File Name Changing

When I send multiple documents at a time for signature they return with the file name changed. For example I send invoices for review and signatures; each document is saved with the invoice number and then sent through Adobe sign for signature. When the approver signs and sends them back all the file names are changed to whatever the Subject line was in the request email. I need to know how to stop this from happening; when I download the file that has been signed I need to attach it in our system with the Invoice number as the file name. I know I can manually change the name back when saving but we do hundreds in a day and this can be very time consuming. PLEASE HELP!