Is this really free, with no account?
Yes. The page is a static file and the PDF is typeset in your browser; there is no server to charge you or to sign you into. The trade-off is the honest one: nothing is remembered between visits unless you save the form as a file.
Why does my total differ from my spreadsheet by a cent?
Rounding order. This invoice rounds each line to the cent and then adds them; a spreadsheet often adds the unrounded lines and rounds once at the end. Both are legitimate, and the rule used here is printed at the foot of the invoice so the client can see which one produced the figure.
Can I put a VAT number, a purchase order or bank details on it?
Yes. Both parties have a tax-number field, and the payment-details block prints whatever you put in it — bank name, IBAN, sort code, PayPal address, a purchase-order reference, payment terms. It is plain text, so anything goes.
Can I add my logo?
Not in this version. The invoice is deliberately plain: a title, a number, the two parties, the lines and the totals, in a layout that prints cleanly on A4 or Letter. A logo needs an image upload and a layout decision, and both are on the list rather than half-done here.
Is my client's information sent anywhere?
No. The form is read by a script in this page, the PDF is typeset by a worker in this tab, and the site has no endpoint that could receive anything. Nothing is stored in the browser either — the site's own local record refuses content by design — so closing the tab is the end of it.