Once logged into the app Navigate to the Product Files page.
This will show a list of the products in your store. Once you’ve found the product you want to upload the file to click manage on the right. This will open a popup which allows you to drag and drop (or click and browse) next files on the left, it will then add to the existing files on the right. Once you’ve clicked on the files you want to attach click on save changes.
These files will now be showing on your product page.