Attach PDF file to Drupal Commerce Order Notification generated with invoice commerce

How do I attach a pdf file to the commerce email?

I have invoice commerce.

Invoice commerce show view pdf option on every order and I want to send that pdf in email

I am stumped on how to get the pdf into a File object.

module_name_mail_alter(&$message) {
 $my_attachment = array(
'filecontent' => $pdf,
'filemime' => $pdf->filemime,
'filename' => $pdf->filename,
'filepath' => NULL,
);
$message['params']['attachments'][] = $my_attachment;
}

I tried below code but It seems there is no table with name commerce_invoice and there is no output

$invoice_storage =\Drupal::entityTypeManager()->getStorage('commerce_invoice');
  $invoice_ids = $invoice_storage->getQuery()
    ->condition('orders', [$order->id()], 'IN')
    ->accessCheck(FALSE)
    ->execute();
$invoices = $invoice_storage->loadMultiple($invoice_ids);

if ($invoices) { 
  $invoice = reset($invoices);
  $vars['invoice_download_url'] = $invoice->toUrl('download');
}