Perhaps you could also have a text submit page (form), and then allow people to download the generated pdf? (Allows people to try out anonymously)

Also, generating in some kind of editable format, may be a good idea. For any minor edits I want to do, on the generated invoice, or fix any errors in generation because of its interpretation of the text.

Agree. Being able to edit is a very useful feature. I'll have to figure out how to make that easier.

I've tried to keep everything thing as simple as possible for now.

Thanks for the feedback.