To change name of regular invoices, simply go to WooCommerce > PDF Invoices > Translation and change the field "invoice_" to anything that suits you. For example, if you change it to "receipt_", regular invoice names will look something like receipt_ABC_123.pdf or receipt_123.pdf if you do not have invoice prefix set up.
Proforma invoices use order ID for their names (e.g. 1259.pdf), so there's nothing to change.
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