When you create a new document configuration (or when you modify an existing one) under Settings > Document and email configuration, all your future documents for the clients for whom you selected this document configuration will automatically use this new/modified document configuration, but your existing documents won't automatically be updated.
And that's logic, because it's too risky to automatically modify a bunch of documents that may already have been sent to clients.
To apply a new document configuration to an existing document, you have to change the document configuration in the document by clicking on Edit in the Billing options panel, and then regenerate the PDF file (= click again on Actions > Generate PDF).
If you added Terms & Conditions to an existing document configuration after having already created documents, you have to click on Edit in the Terms part of the document, and then on 'Reset the terms'.
