Sign puts a picture of your signature on the page — the same as signing a printout and scanning it back. Under the EU’s eIDAS regulation that is a simple electronic signature: a court may not dismiss it only because it is electronic, but nothing in the file proves who placed it, and the document can be changed afterwards without a trace. A qualified electronic signature (QES) is a different thing: it is made with a certificate issued to you by a qualified trust service provider after checking your identity, and it is cryptographically bound to the document, so any later change shows. Only a QES has the legal effect of a handwritten signature everywhere in the EU. This page does not make one and cannot. If a contract, a notary, a bank or an authority asks for a qualified or advanced signature, use the service they name — in Luxembourg, for example, LuxTrust.
The PDF never leaves this tab: the browser draws its pages and writes the signed copy itself. No upload, no account. The original pages are left as they are; your signature, date and text are added on top.