Example on how add one or more extra files into the whole zip archive.

bulk-download-for-gravity-forms-add-extra-files-after-files-added-to-zip.php

<?php
/**
 * Plugin Name: Bulk Download for Gravity Forms - Add one or more extra files into the whole zip archive.
 * Description: Example on how add one or more extra files into the whole zip archive.
 * Author: VCAT Consulting GmbH
 * Author URI: https://www.vcat.de
 */


/**
 * Action to add one or more extra files into the whole zip archive.
 *
 * @param ZipArchive $zip The Zip Archive.
 * @param array  $uploaded_files All uploaded files.
 * @param array  $form The form array.
 */
function my_bdfgf_extra_files_after_all_files_zipped( $zip, $uploaded_files, $form ) {
	$wp_upload_dir = wp_upload_dir();

	/*
	 * Adds an extra file in zip archive after all uploaded files and folders have passed through the mergetag filter.
	 *
	 * Extra file must be on the server!
	 *
	 */
	$zip->addFile($wp_upload_dir['basedir'] .'/extra_files/information.pdf','new.pdf');
}

add_action( 'bdfgf_after_uploaded_files', 'my_bdfgf_extra_files_after_all_files_zipped', 10, 3);