Invoice number to be the same as Order number , here is the code for this one.

/**
* Format order number with invoice number settings
*/
add_filter( 'wpo_wcpdf_raw_document_number', 'wpo_wcpdf_raw_document_number', 10, 4 );
function wpo_wcpdf_raw_document_number( $number, $settings, $document, $order ) {
if ( $document->get_type() == 'invoice' ) {
$number = $order->get_order_number();
}

return $number;
}