I would suggest to have localized order.pdf name so that users can identify that as something meaning full. Also the number might be formatted for me with trailing zero's and or a prefix.
J.
Sample code below [not tested yet]. No number prefix implemented since it would touch too much code. But for pdf done.
var orderPlacedAttachmentFilePath = _orderSettings.AttachPdfInvoiceToOrderPlacedEmail ?
_pdfService.PrintOrderToPdf(order, 0) : null;
var orderPlacedAttachmentFileName = _orderSettings.AttachPdfInvoiceToOrderPlacedEmail ?
string.Format("{0}-{1}.pdf", _localizationService.GetResource("QE.Customer.Pdfname", order.CustomerLanguageId), order.Id.ToString("D10")) : null;
int orderPlacedCustomerNotificationQueuedEmailId = _workflowMessageService
.SendOrderPlacedCustomerNotification(order, order.CustomerLanguageId, orderPlacedAttachmentFilePath, orderPlacedAttachmentFileName);
if (orderPlacedCustomerNotificationQueuedEmailId > 0)