What this actually is, stated plainly
This puts a picture of a signature onto a page. That is the whole operation. The image is embedded in the PDF and flattened into the page content, exactly as a logo or a photograph would be.
It is not a digital signature. There is no certificate, no key pair, no timestamp authority and no identity check. Nothing in the resulting file records who placed the image, when, or from where. Nothing detects whether the document was altered after the image was added. Anyone who receives the file can copy the signature image out of it and put it on a different document in about a minute, and the result will look exactly as convincing as yours.
A great many free signing sites imply otherwise — "legally binding", "secure e-signature", a padlock in the interface — while doing precisely what this does. The honest version is worth saying: for a huge number of everyday cases, a picture of a signature is completely fine and is what the other party expects. For the cases where it is not fine, no browser tool that never asked who you are could possibly be enough.
What a real digital signature is, and when you need one
A cryptographic digital signature is a different mechanism entirely. A certificate issued to you by a certificate authority, after that authority has verified your identity, is used to compute a hash of the document and encrypt it with your private key. That signature is embedded in the PDF along with the certificate chain. The standard for doing this in PDF is PAdES, built on the older CAdES and PKCS#7 work.
The result is checkable. A reader can recompute the hash and confirm the document has not changed by a single byte since signing; it can verify the certificate chain up to a trusted root; and depending on the profile it can prove the signature existed at a particular time, and remain verifiable years later. Alter one character of the document and the signature visibly breaks.
You need that when the stakes justify it: property transactions, regulated filings, anything under eIDAS in the EU where a qualified electronic signature carries the legal weight of a handwritten one, tenders, and any agreement where a counterparty might later dispute what they agreed to. Getting one means going to a qualified trust service provider or a certificate authority and proving who you are. It cannot be done for free, in a browser tab, by a site that has never met you.
Three ways in, and why the background matters
You can draw the signature with a pointer — a finger and a stylus on a touchscreen, a trackpad, or a mouse all take the same path, and the surface refuses to scroll while you are drawing on it. You can type a name, which is set in this site’s own self-hosted typefaces on a slant; they are not handwriting fonts, because shipping one would mean fetching a font from a third party, which no page here does.
The third way is the common one: a photograph or scan of a signature you wrote on paper. That image arrives as dark ink on a light background, and the tempting shortcut is to paste it in and let the white rectangle cover whatever is underneath. That works on a blank white page and fails everywhere else — on a shaded form, over a printed signature line, next to a table border, on a coloured letterhead.
So the paper is removed rather than hidden. Every pixel is composited onto white, its brightness is measured, and that brightness sets its new transparency through a soft ramp: dark strokes stay fully opaque, paper disappears entirely, and the values in between become a gradient, which is what keeps the edge of a pen line smooth instead of jagged. The result is then cropped to the ink, so the box you drag is the signature rather than three inches of desk. A slider adjusts the threshold when the scan is grey or the pen was light.
Placing it, and the page it lands on
Drag the signature where it belongs and pull the corner handle to size it. The aspect ratio is held in points rather than in page fractions, so it does not stretch when the page is not square, and the position you see is the position that is written — the preview and the export share the same geometry rather than approximating each other.
Pages that carry a rotation are handled properly, which is worth checking in any tool you use for this. A scanned page frequently stores itself upright and tells the reader to display it turned; place a signature on it with a naive transform and it lands on the wrong edge, or upside down, or on the correct edge of the wrong page. Here the placement is converted once, against the real page box and the real rotation, at the moment the file is written.
When you are done, the whole document is rebuilt in one pass with the signature embedded as a transparent PNG. Your original file is never touched. And as with everything else on this site, the PDF is read, drawn on and rewritten inside this browser tab — which for a document you are about to sign is not a minor detail.