nf_email_attachment_filter.php

<?php

add_filter( 'nf_email_notification_attachments', 'my_custom_attachment', 10, 2 );

function my_custom_attachment( $attachments, $action_id ){

    // Confirm the Action ID is the intended email action.
    if( '1' != $action_id ) return $attachments;
  
    // Create Attachment File Path
    $upload_dir = wp_upload_dir();
    $path = trailingslashit( $upload_dir[ 'path' ] );
    $file_path = $path . 'my_custom_attachment.pdf';

    // Push File Path to attachments array
    $attachments[] = $file_path;

    // Return the updated attachments array
    return $attachments;
}