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Thanks for responding Phong. Just curious then in the scenario I described, would I then have to just cache the pdf file on the server in a file, then fopen that file and send the fax with the attached pdf file with the code you sent me. That's really the only way then?

Unfortunately, the fax API does not support base64 encoded (the sample code in the API reference is incorrect). This is the only way how you fax attachments.

...
$request = $rcsdk->createMultipartBuilder()
                 ->setBody(array(
                     'to' => array(array('phoneNumber' => $RECIPIENT)),
                     'faxResolution' => 'High',
                 ))
                 ->add(fopen('log.txt', 'r'))
                 ->add(fopen('test.html', 'r'))
                 ->add(fopen('test.jpg', 'r'))
                 ->add(fopen('test.pdf', 'r'))
                 ->request('/account/~/extension/~/fax');

$resp = $platform->sendRequest($request);
print_r ("FAX sent. Message status: " . $resp->json()->messageStatus);