In order to get a document from seller center it's required to specify a type, which can be 'invoice', 'exportInvoice', 'shippingLabel', 'shippingParcel', 'carrierManifest', or 'serialNumber' and the order item IDs.
The following method returns a collection of invoices:
$documentType = 'invoice';
$orderItemIds = [1234, 4567, 890];
// Get invoice of order item #1234, #4567 and #890
$document = $sdk->documents()->getDocument($documentType, $orderItemIds);