You can enable customers to attach files to products they want to order after configuring the Customer Files Uploads & Product Attachments add-on.
For this purpose:
On the product listing page of your store's Admin area (Catalog -> Products), locate a product and open its product details page as described here.
On the product details page, enable the "Allow buyers to attach files to this product" toggle.
Specify whether the file attachments should be mandatory for the product or not:
NOTE: If you set product attachments mandatory, a customer can 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.
Buyers will have access to the file attachments widget both on the product quick view page and on the product details page:
The files attached will be registered and displayed on the cart page and in the customer invoice:
The store administrator can check and download the files a customer attached to the product on the order details page in the store's Admin area (Orders -> Order list):