Dropping a fillable PDF here surfaces its fields as an ordinary web form: text boxes to type into, checkboxes to tick, dropdowns to choose from. Fill it in, then save either a normal PDF that stays editable or a flattened one where the answers become permanent page content. Flattening is the option most people actually want when returning a form to someone: it stops the answers from being changed accidentally by the next reader, and it guarantees they display in software that renders form fields badly or not at all. The work happens in your browser, so a completed application containing your address, salary or medical details is never uploaded anywhere. If the PDF has no fields — which is true of most scanned forms — the editor tool is the fallback, adding text boxes wherever they are needed.
Fillable and flat forms are different things
A fillable PDF contains AcroForm fields: named, typed objects the reader knows how to accept input into. A flat form is a picture of a form, whether scanned from paper or exported without the interactive layer, and has nowhere to put anything.
Most forms encountered in the wild are flat, which is why filling one so often ends in printing, writing and scanning. The way out is to add text boxes over the top with the editor: the result looks the same to whoever receives it, and does not require the sender to have produced a proper form.
What flattening does
Flattening draws the field values into the page and removes the interactive fields. The answers stay visible and printable but are no longer editable, and no longer depend on the recipient's reader supporting form fields — which some mobile viewers and browser previewers handle poorly.
The cost is that it is one-way. Keep the un-flattened version if the form may need revising, because recovering editable fields from a flattened document is not possible: the fields are gone and only their rendered values remain.
Filling forms with personal information
Forms are, by their nature, the documents most likely to contain exactly what you would not want on someone else's server: identity numbers, financial details, medical history, addresses. Most online form fillers upload the file to do the work.
This one does not. The PDF is parsed and rewritten entirely in the browser tab, so the completed form exists only on your machine until you send it wherever it is going. That property is the reason to prefer a local tool for this particular job, more than for any other in this set.