Signing a document: what we prepare, and what your own software puts in

A seal on a PDF sits inside the file itself, and the reading software checks it on its own. The engine prepares the document and leaves exactly the room the seal goes into. The seal itself is made by your own software, from your own certificate, and never leaves your machines.

You have been sent a contract and you want to send it back signed

That is the everyday case, and it splits cleanly in two. Adding to the document — a signature area, a date, a line of your own, a page at the end — is work on the file itself, and we do it for you wherever the document comes from: one you wrote, one somebody sent you, one that came out of software you have never opened. Everything already in it is left exactly as it was, to the byte.

Sealing it is the other half, and that half stays with you. The seal is made on your own machines, by your own software, from the signing key that is yours alone. You get the finished file back and you seal it there.

That division is worth having, and it is worth having for you. A signing key is the one thing that cannot be replaced if it is taken: whoever holds it can sign in your name, and no message afterwards undoes a document already sent. A signing key we never hold is one nobody can take from us. It stays where you put it, on your machines, under your own lock.

And the file itself travels either way. Sending us a document to add to is no different from sending us one to write: it is the private key, and only the private key, that stays behind.

A scanned signature proves nothing at all

Anybody can lift a picture of a handwritten signature off one document and drop it onto another. Nothing in the file says who did it, and nothing says whether an amount was altered afterwards. Whoever opens it sees a picture, and a picture is all it is.

An electronic signature is a different thing entirely. It is worked out from every single byte of the document. Change a comma, a date or a figure, and the seal no longer matches: the reading software says so in a banner across the top of the page, before anyone has read a word.

The two things a signed document proves

• Who sent it. The seal names the company or the person behind the document, and an authority checked that name before issuing the certificate the seal is built on.
• That nobody has touched it. Not one byte has moved since it was sealed: not the amount, not the date, not the small print at the bottom.

That is what makes a signed contract or invoice worth sending. The person receiving it does not have to take your word for it, nor trust the email it arrived in.

European law says as much. Under the European regulation on the electronic signature, in force since 2014, a signature may not be refused in court merely for being electronic, and the strongest kind is worth as much as a signature written by hand.

What a certificate is, and why it is yours and not ours

A certificate is your identity written in a form a machine can check. You get one from a body whose trade is verifying who you are: the same idea as an identity card, issued by somebody trusted rather than printed at home. It comes with a private key — the one thing that makes a seal yours, and the one thing you must never hand to anybody.

Your certificate is never handed to us. At the moment of sealing, the engine passes the document to your own software and writes back whatever comes out.

The fiddly step, and what the engine takes off your hands

Sealing a PDF is awkward in a way that has nothing to do with your business. The seal has to sit inside the very file it covers, so the file must state, byte by byte, which stretches of itself the seal accounts for and which single stretch it does not: the hole the seal itself goes into. One byte out either way and reading software everywhere rejects the document, with a message nobody can do anything about.

The engine does that step. It reserves the room, works out the exact extents, hands your software the whole file bar that hole, and writes the answer back in place without moving a single byte.

• A signature area placed on the page like any other box, with a frame around it — or no area at all, when the seal is meant to leave no mark.
• The seal in either of the two forms the PDF standard allows. Ask for the newer one when the document goes to a public body, since that is the form European requirements are written around; the older one is what most software has always written, and it is the safe choice everywhere else.
• Sealing a PDF that already exists, even one written by somebody else's software: the original file is kept exactly as it was, and the seal is added after it.
• A second signature after the first, so two people can approve the same document without the first seal being disturbed.
• A seal that also states what may still be changed afterwards: nothing at all, filling in the boxes of a form, or adding notes.

What it does not do, and why that is the right answer

The certificate comes from the authority you chose, and the seal is made on your own machines by your own software. A certified time on that seal — what a customer asks for when a signature must still be proved good years later — comes from the same supplier: it is theirs to give. And the division is on purpose: your private key stays exactly where you put it, because we never ask you for it.

Whether the finished document satisfies the European format down to its last requirement depends on the seal your own software produces, and that is not ours to promise.

Where this fits

Adding to a document is offered everywhere the engine is: inside your own program with the library, on your own machines with the server, and through the service we run. The document goes over, comes back with what you asked added, and everything that was already in it is untouched.

Sealing is the one step we never do for you, wherever you run us from, because it is the one step that would mean holding your signing key. You seal the file you get back, on your own machines, with your own software.

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