Why invoices should always go out as PDF
An editable Word or Excel invoice invites accidental (or not-so-accidental) edits after the fact, and doesn't render consistently across accounting software. PDF is the de facto standard for finalized financial documents because the numbers can't be casually altered and the layout is guaranteed to match on both ends.
Since invoices carry client names, amounts, and sometimes banking details, converting them locally in your browser instead of through an upload-based tool means that financial data never passes through a third-party server on its way to becoming a PDF.