Tools
Use it before you decide anything.
Two kinds of thing live here. Tools that do the job on the spot, with no account and no email field, and the product they are a small piece of. There is one tool today, which is fewer than we would like and more honest than a page pretending otherwise.
Invoice generator
Fill in your details, the client, the line items and your logo, and get a finished invoice as a PDF. No account, no email, and nothing you typed is kept: the document is rendered and handed to you, and that is the end of it.
If you send more than a couple of these a month, the generator stops being the right tool.
That is when an invoice wants to remember who owes you what.
Do I have to sign up to use the invoice generator?
No. There is no account, no email field and no trial. The form is on the page, the PDF comes back, and we keep nothing you typed into it.
Is the invoice legally valid?
It carries what an invoice normally carries: your details, the client's, dated line items, totals and a reference. Whether that satisfies a particular country's rules is a question for that country, and for anything involving tax registration numbers you should check what yours requires.
What do I get by signing up instead?
The generator makes a document. An account makes a record: what was sent, what was paid, what is late, reminders that go out on their own, and the income booked when the money arrives. If you invoice occasionally the document is enough.
When the document is not enough.
An account remembers who owes you what, and chases them for you.