How to sign a PDF

What each one actually proves

An electronic signature

A picture of a signature, placed on the page. It proves nothing about identity on its own — anyone can draw a name — and it does not detect later changes. Its legal weight in most jurisdictions comes from the surrounding evidence: the email trail, the timestamps, the parties' conduct. That is usually enough, which is why the great majority of signed documents work this way.

A digital signature

A cryptographic operation using a certificate issued to a person or an organisation. It binds the signature to a specific byte sequence, so a reader can verify two separate things: that the certificate belongs to who it claims, and that the document has not been altered since signing. Change a single character and verification fails.

Frameworks such as the EU's eIDAS define tiers of exactly this, with the higher ones requiring a qualified certificate from an accredited provider. If you have been asked for a "qualified" or "advanced" signature, that is a certificate you obtain from such a provider — not something any editing tool can produce.

Adding a signature

Sign PDF adds an electronic signature: draw it, type it, or upload an image of one, then place it where it belongs. It runs in your browser, so a contract you are signing is never uploaded anywhere.

Some practical points that matter more than they sound:

• Sign the final version. Any later edit — even adding a page — changes the document you signed.
• Fill the fields first. If the document is a form, complete it before signing; fill in a PDF form covers that, and how PDF forms work explains why the order matters.
• Flatten it afterwards. Flatten PDF merges the signature into the page so it cannot be dragged off or deleted in a reader, and so it renders identically everywhere.
• An uploaded signature image is reusable by anyone who receives it. A clean scan of your handwritten signature on a white background is, in effect, a credential. Be as careful with that file as you would be with anything else that can act as you.

Making a signed document hold up

Whichever kind of signature you use, the document around it does some of the work.

• Keep the audit trail. The covering email, the timestamps and the version that was agreed are frequently what settles a dispute, not the mark on page twelve.
• Check the metadata before sending. A signed contract carries author names, the producing software and sometimes revision history. The metadata editor shows what is there and what PDF metadata contains explains why it is worth a look.
• A password is not a signature. Encryption controls who can open a document; it says nothing about who agreed to it. What a PDF password actually protects sets out the difference.
• Do not rely on a watermark or permission flags to stop alteration. Neither prevents editing — see how to watermark a PDF.

This is general information about how the technology works, not legal advice. Requirements for a valid signature vary by country and by contract type, and if a document genuinely matters, that is a question for a lawyer rather than a tool.