If you have enabled to create PDF Invoices automatically for all order in WooCommerce you or the customer can see & download them in the following ways:
In WP-Admin > Order > Overview you can download invoices and packing slips directly. Furthermore you can bulk download multiple invoices.
For each order you will see a small icon on right actions column > download invoice.
In WP-Admin > Order > Single Order you can view and download invoices. Invoice PDFs or packing slips can also be created manually or updated.
My Account & Thank you
Customers can see invoices in order details or thank you page. From there your customers can download their invoice from the single order details page or the thank you page, that comes after complete checkout.