API requests are made through a set of proxy classes you could find under Emis\Document\Api\Proxy namespace. A simple example getting document titles for country code 'HU':

require('vendor/autoload.php');

use Emis\Http\Client;
use Emis\Document\Api\Proxy\Search;

$client = new Client();
$client->setApi(Client::DOCUMENTS_API);
$client->setToken("YOUR_TOKEN");

$search = new Search($client);
$result = $search->query('HU');

foreach ($result->getDocuments() as $document) {
    echo $document->getTitle() . "\n";
}