Document document = new Document();

OrderCollection orders = ShopServiceManager.Orders().GetOrdersByCustomer(34);
foreach (var order in orders)
{
    BL.Documents.Invoice invoice = new BL.Documents.Invoice();
    invoice.DocumentCreator = new InvoiceDocument();
    invoice.Order = order;
    invoice.Print(false);
    document.Add(invoice.Document.LastSection.Clone());
}

PdfDocumentRenderer pdfRenderer = new PdfDocumentRenderer(true);
pdfRenderer.Document = document;
pdfRenderer.RenderDocument();