To download a file attached to Document, DocumentRevision or Note, use Files::download() method.
$files = $client->files();
$result = (string)$files->download('Document', '2f5d6dec-e1c5-e7c1-33f9-5a9119971946');
echo json_encode($result, JSON_PRETTY_PRINT);
1 "This is the contents of text file!!!\n"
Files::download() returns an instance of GuzzleHttp\Psr7\Stream
class. You can simply cast the stream to a string, as in example above. If you want to save to a file, you can pass an additional argument to Files::download():
$files = $client->files();
$files->download('Document', '2f5d6dec-e1c5-e7c1-33f9-5a9119971946', '/tmp/destination.txt');
$fh = fopen('/tmp/destination.txt', 'w');
$files->download('Document', '2f5d6dec-e1c5-e7c1-33f9-5a9119971946', $fh);