How can I display or download order invoices from back end ?
I assume this is a question about Commerce? I’ve moved your topic into that category.
When viewing an order you can click on the invoice number shown in the metadata at the top to download it. There’s not currently a built-in list of all invoices in the dashboard (we can look at adding that in 1.3 if you’d like such a list), but you can also find them in core/export/invoices/{year}/ if you just need to download them all.