The Page Designer App can create some nice looking documents, but it is a manual process to generate each PDF.
If you’re looking for a formal PDF invoice that is an attachment to the email you send, you’re most likely going to have to use some sort of external application that will generate the PDF.
There is an official FormStack App in the marketplace.
Or you can use a series of Zapier/Integromat automation along with a myriad of other document generation software such as https://www.pdfmonkey.io/.
On the other hand, if you are okay with a super basic invoice that is just text-based in an email, the solution becomes easier as you can use some of the built-in functions/tools in Airtable. You can use the built-in Airtable Automations to send out the emails.