When you add new documents to a product, they becomes visible on the client website for the buyers, in the specific product page(s). The buyers are not announced of the changes automatically via email and do not receive those documents via email, if that is what you meant. But, if they access the product content as logged clients, they will see the additions.
To announce the buyers group of the changes, you can send an email to that particular buyers group (from Communicate > Emails). You can tell them something like "I added this page and this document, etc". If you still have questions that concern a certain product, you can submit a ticket in the Help Desk where you specify the product and the modifications to it, and we will look further into the matter.