How can I configure the PDF file prefix?

Proforma invoices use Order IDs for document number / file name. Proforma invoice file name would look something like 59.pdf.

Regular invoice file name would look something like invoice_xxx_111.pdf. You can change the “invoice_” part under WooCommerce > PDF Invoices > Localization. You can change the “xxx” part under WooCommerce > PDF Invoices > Settings (field “Invoice number prefix”).