By default the WooCommerce PDF Invoices & Packing Slips plugin restricts the view access for PDF documents to only users with manage_woocommerce capabilities or customers from the My Account page in the frontend.
But every case is different and sometimes you need to display a button, or a link, for the document outside this parameters which requires a different approach. You might need to display the Invoice link in the thank you page or in an email, to mention two cases where you can use this setting.
This is the ultimate setting to allow visitors or other guest users to access a PDF document without requiring to be logged in or having an account in your store.
You could enable this setting under your plugin Status tab, like below:
The technique behind this setting is simple, instead of relying on nonces, the plugin generates a unique key for the document order. If the order unique key corresponds to the order, the user is allowed to see the PDF document