Jan - thanks for the input, I was finally able to get to the bottom of this and sharing my outcome below for anyone else that may end up in the same position.
The AWS PHP SDK utilizes Guzzle for handling the request and response to Lambda, so in the response the actual payload is private/hidden unless you use the mechanism provided by the SDK to extract it from the response. Below is an example of how this was handled
$client = LambdaClient::factory(array(
'version' => 'latest',
'region' => 'us-east-1',
'credentials'=> array(
'key' => getenv('AWS_ACCESS_KEY_ID'),
'secret' => getenv('AWS_SECRET_ACCESS_KEY'),
)
));
$result = $client->invoke(array(
'FunctionName' => 'jsfunction',
'InvocationType' => "RequestResponse",
'Payload' => $data, // your json payload
));
// Get the payload from the object
$body = json_decode($result['Payload']);
// Base64 Decode the PDF/excel data
$pdf_data = base64_decode($body->body);