How to Add a Digital Signature to a PDF
The term 'digital signature' actually covers two quite different things, and knowing which one you need matters more than most people realize.
Electronic signature vs. true digital signature
An electronic signature is typically a drawn, typed, or uploaded image of your signature placed onto the page. It's fast and works for most everyday documents, but it doesn't cryptographically prove who signed the file or verify the document hasn't been altered afterward. A true digital signature uses a certificate-based cryptographic key, which does both of those things, and is what's required for many legal, government, or high-stakes business documents.
Adding a simple electronic signature
Upload your PDF, draw your signature with a mouse or trackpad, type your name and pick a script-style font, or upload a photo of your handwritten signature on paper. Then place it on the appropriate line and save. This works well for informal agreements, internal approvals, and most day-to-day paperwork.
Adding a certificate-based digital signature
This requires a digital certificate, either from a certificate authority or generated within signing software, and produces a signature that can be verified cryptographically and will show a warning in PDF readers if the document is altered after signing. This is the appropriate choice for contracts, legal filings, and anything where you need to prove authenticity later.
Multiple signers
If a document needs several people to sign, decide upfront whether you need them to sign in a specific order or simultaneously, since some documents (like approvals that depend on an earlier signature being valid) need sequential signing while others don't. Track which version of the file each signer is working from to avoid signatures ending up on outdated copies.
