Use the js-cookie API to set a cookie when download page loads

add_js_cookie_enqueue_script.php

<?php // Ignore this first line when copying to your child theme's functions.php file.

/** 
 * Example of enqueing js-cookie if needed. 
 *
 * Learn more about js-cookie https://github.com/js-cookie/js-cookie
 */

function add_js_cookie_enqueue_script() {
	// Add to the head section.
	wp_register_script(
		'js-cookie-3-0-1', 
		'https://cdn.jsdelivr.net/npm/[email protected]/dist/js.cookie.min.js',
		array(), 
		'3.0.1', 
		false
  );
  
  wp_enqueue_script('js-cookie-3-0-1');
}
add_action('wp_enqueue_scripts', 'add_js_cookie_enqueue_script');

set_cookie_for_download_page.php

<?php // Ignore this first line when copying to your child theme's functions.php file.

function set_cookie_for_download_page() { ?>
 	<script type="text/javascript">
		jQuery(document).ready(function ($) {
			
			// 1) Change the URL of the thank you page to your thank you page.
			// 2) Change the cookie name to your popup's cookie name.
			// 3) Change the expiration time to your expiration time.

			// 1) Bail if we're not on the download thank you page.
			if (document.URL !== 'https://twenty-twentyone.local/your-download-file/') return;
			// Bail if the JavaScript Cookie API isn't available.
			if (typeof Cookies.set !== "function") return;
			// 2) and 3) Use JavaScript Cookie (js-cookie) API to manually create a cookie for a form submission.
			Cookies.set('pum-1604-form-submitted', 'true', { expires: 1, path: '/' }); // Expires in 1 day.
			
    		}); // jQuery
 	</script>
<?php }
add_action( 'wp_footer', 'set_cookie_for_download_page', 500 );
/**
 * You can add the PHP code snippet to your child theme's functions.php file 
 * or with third-party plugins such as My Custom Functions and Code Snippets.
 * 
 * Learn more:
 * - https://docs.wppopupmaker.com/article/84-getting-started-with-custom-js
 * - https://docs.wppopupmaker.com/article/552-getting-started-with-custom-php
 */