How to get (download) Invoice PDF by API (web services)?

I'm a newbie in NetSuite, today I'm trying to develop a demo to get invoice. I used this library

ryanwinchester/netsuite-php

I can get invoice information by this code:

$transactionSearch = new SearchEnumMultiSelectField();

$transactionSearch->operator = "anyOf";

$transactionSearch->searchValue = array('_invoice');

$search = new TransactionSearchBasic();

$search->type = $transactionSearch;

$request = new SearchRequest();

$request->searchRecord = $search;

$searchResponse = $netsuite_service->search($request);

But no invoices PDF URL in response.

Then I tried with MessageSearchBasic

$messagesSearch = new SearchBooleanField();

$messagesSearch->searchValue = true;

$search = new MessageSearchBasic();

$request = new SearchRequest();

$request->searchRecord = $search;

$searchResponse = $netsuite_service->search($request);

I can get the list of sent email with invoices PDF URL, but this one only can get sent email, for transactions not yet send email this can't get.