Thank you so much for your help !
I have the shared link from DropBox entered into my WooCommerce (on WordPress) for the PDF product in my store. I have other products all done the same that are working normally.
Today one of my products was featured on another site with a link directly to my bookstore. People are most likely clicking on that link to my store, and then downloading the product directly from my site (from both mobile devices and laptops). I can't even get it to work on my own computer.
If I put the copied link into the browser it opens fine.
It was previously working but isn't anymore.
This is the error message-