Modify the font size of the text in the Product Table and Summary Table of the Invoice document - By WebToffee ( WooCommerce PDF Invoices, Packing Slips, Delivery Notes, and Shipping Labels)

functions.php

<?php //do not copy this line of code
add_filter('wf_pklist_alter_template_html', 'wt_pklist_change_product_table_body_bg_font', 10, 2);

function wt_pklist_change_product_table_body_bg_font($html, $template_type) {
    if ('invoice' === $template_type) { // or 'packinglist' if needed
        $html .= '<style type="text/css">
            .wfte_product_table_head th {
                font-size: 18px; /* Font size for header */
            }
            .wfte_product_table_body td,
            .wfte_payment_summary_table_body td {
                font-size: 16px; /* Font size for body */
            }
        </style>';
    }
    return $html;
}