yellyfish

Ewout,

Great, thanks for the input.
I already sorted this out, which works great.

//Generate an email attachment filename for wpo_wcpdf_invoice_number
add_filter( 'wpo_wcpdf_attachment_filename', 'my_pdf_attachment_filename', 10, 3 );
function my_pdf_attachment_filename( $filename, $display_number, $order_id ) {
    //$display_number is either the order number or invoice number, according to your settings
    global $wpo_wcpdf;
    $invoice_number = $wpo_wcpdf->get_invoice_number();
    $filename = 'Factuur_' . $invoice_number . '.pdf';

    return $filename;
}