Change Invoice Number to Receipt Number

Hello @padroang

Please add the following code snippet to your theme functions.php file:

add_filter( 'gettext', 'wcpdf_override_strings', 10, 3 );
function wcpdf_override_strings( $translation, $text, $domain ) {
	if( $domain == 'woocommerce-pdf-invoices-packing-slips' ) {
		if( $text == 'Invoice Number:' ) {
			$translation = 'Receipt Number:';
		}
		if( $text == 'Invoice Date:' ) {
			$translation = 'Receipt Date:';
		}
	}
    return $translation;
}

If you never worked with actions/filters please read our documentation page: How to use filters