Download link on the Thank You page

You can add a download link to the PDF invoice on the WooCommerce Thank You page with a small code snippet (filter). Note that due to security restrictions, this will only work for registered/logged in users!
Read this if you haven’t used wordpress filters / edited functions.php before!

add_filter('woocommerce_thankyou_order_received_text', 'wpo_wcpdf_thank_you_link', 10, 2);
function wpo_wcpdf_thank_you_link( $text, $order ) {
    if ( is_user_logged_in() ) {
        $order_id = method_exists($order, 'get_id') ? $order->get_id() : $order->id;
        $pdf_url = wp_nonce_url( admin_url( 'admin-ajax.php?action=generate_wpo_wcpdf&template_type=invoice&order_ids=' . $order_id . '&my-account'), 'generate_wpo_wcpdf' );
        $text .= '<p><a href="'.esc_attr($pdf_url).'">Download a printable invoice / payment confirmation (PDF format)</a></p>';
    }
    return $text;
}

alternatively, you can hook this text to the woocommerce_thankyou action, see this thread on the support forum.