When you complete the "Customer Files Uploads & Product Attachments" add-on setup, proceed to enable customer file attachments for your products.
For this purpose
On the product listing page of your store's Admin area (Catalog -> Products), select a product and open its product details page as described here.
On the product details page, enable the toggle "Allow buyers to attach files to this product."
Specify whether the file attachments should be mandatory for the product or not:
Note that if you set product attachments mandatory, a customer will not proceed to checkout without adding a file to the product they want to order.
Save the changes.
That's it. File attachments for the product are now enabled.
Your customers will be able to access the product file attachments widget both on the product quick view page and on the product details page:
Product details page
Quick view page
The files attached will be registered and displayed on the cart page and in the customer invoice:
The store administrator will be able to check and download the files attached to the product by a customer via the order details page (Orders -> Order list):