forminator_email_attachment.php

<?php
add_filter('forminator_custom_form_mail_admin_headers', 'wpmudev_change_forminator_header_attachment', 10, 5);
function wpmudev_change_forminator_header_attachment($headers, $custom_form, $data, $entry, $cls)
{
    $form_list = array(1048104, 1045, 1050);//Please put here the formIDs

    if (!in_array($custom_form->id, $form_list)) { 
        return $headers;
    }

    $headers[] = 'X-FORMINAT-ATTCH : true';
    $headers[] = 'X-FORMINAT-FORM-ID :'.$custom_form->id;

    return $headers;
}

add_filter('wp_mail', 'wpmudev_attach_pdf_forminator', 10, 1);
function wpmudev_attach_pdf_forminator($args)
{
    //Please, here put the list of attachmantes, one per line and the format is: FormID => ATTACHMENT-FILE-PATH,
    $attachment_list = array(
        1048104 => WP_CONTENT_DIR . '/uploads/2023/01/333333333333333333-English.pdf',
        1045 => WP_CONTENT_DIR . '/uploads/2023/05/ISAM-Sales-Leadership-Brochure-1045.pdf',
        1050 => WP_CONTENT_DIR . '/uploads/2023/05/ISAM-Sales-Leadership-Brochure-1050.pdf',
    );
    
  
    if (is_array($args['headers'])) {
        foreach ($args['headers'] as $key => $value) {
            if (strpos($value, 'X-FORMINAT-FORM-ID') !== false) {
                $formID = explode (":", $value); 
                $args['attachments'][] = $attachment_list[intval($formID[1])];
            }
        }
    }
    return $args;
}