Full tutorial on how to use this code here: https://gravityranger.com/attaching-a-file-to-your-gravity-forms-notifications/

notifications_attach_file.php

<?php
// Do NOT include the opening php tag
// Replace 1 with your forms ID
// Replace User Notification with the name of your notification
// Insert your file path

add_filter( 'gform_notification_1', 'add_notification_attachments', 10, 3 );
function add_notification_attachments( $notification, $form, $entry ) {
    if ( $notification['name'] == 'User Notification' ) {
        $path = 'path/to/file.pdf';
        $notification['attachments'] = array( $path );
    }
    return $notification;
}