The guest users do not have print or download permission. The customer needs to log in to the site to achieve it. To validate is done for security purposes.
However, you can enable the “Attach invoice PDF in order email” option located on the invoice general tab and set desired statuses to generate invoices. This will add the PDF of the invoice to the order status emails . By this way, you can download invoice PDF without a user account. You can go through this article for a better understanding.
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