Hi @sadiqmayari,
This is because a PDF invoice was not created for those orders yet. Since you are not creating PDF invoices automatically through the email notifications, you have to create the PDF invoices manually, using the buttons in the order list:
…or the buttons in the order details, within the Create PDF panel:
Just to clarify, the way our free plugin creates an invoice automatically is when you select order email notifications in which it will be attached, under WooCommerce > PDF Invoices > Documents > Invoice > Attach to: The attachment process will create an invoice document with its respective number:
In the Professional extension, there is another way to create documents, based on the order status (without the need to attach it), specifically the Create automatically for setting. That is, if you select the Processing or Completed order status for this setting, the PDF document will be created only when the order reaches any of these order statuses:
As you may have noticed in both examples, I have used the Processing and Completed order statuses, because these are the ones I recommend selecting to generate PDF documents only in orders where you have received a payment.