Orders >> Orders and open the order.
In the product list, every customized line shows a PrintApp Customization
block with the customer’s design previews and download links for the print-ready
PDF, PNG, JPEG and TIFF files, plus a Modify Project link that opens the
project in your Print.App admin.
On the PrestaShop 1.7 build the TIFF link is not available. Download the TIFF
from the project in your Print.App admin instead.