How to Sign a PDF Electronically (And the Difference Nobody Explains)
Two things get called "signing a PDF" and they are worlds apart in what they prove. One is a picture of your name. The other is cryptography that will tell you if a single character changed after you signed. Most people use the first and assume they got the second.
The two mechanisms
An image-based signature
You draw with a mouse or finger, type your name in a script font, or upload a photo of your handwritten signature. The result is placed onto the page as an image or as drawn paths.
Mechanically this is no different from any other image in the document. It carries no information about who put it there, when, or whether the surrounding text has changed since.
Its evidential weight comes entirely from context: the email thread it arrived in, the timestamps, the conduct of both parties. Which is often enough for routine agreements, and thin if things turn adversarial.
A cryptographic digital signature
This uses a certificate and a private key. Signing computes a cryptographic hash of the document's exact contents and encrypts that hash with your private key, embedding the result in the file.
Anyone can then verify two distinct things:
- Integrity — recompute the hash and compare. Change one character anywhere and it will not match. The reader reports the document as modified since signing.
- Identity — the certificate identifies the signer, and if issued by a recognised certificate authority, that identity has been verified by a third party.
This is why a digitally signed PDF opens with a banner saying the signature is valid and the document has not been modified. No image-based signature can produce that, because there is nothing to verify.
Which do you actually need?
An image signature is usually fine for internal approvals, low-value routine agreements, acknowledging receipt of something, or anywhere both parties already trust each other and the amount at stake is small.
A digital signature is worth the setup for high-value contracts, regulated or audited processes, anything with a real chance of dispute, and situations where the recipient specifically asks to verify who signed.
The honest middle ground: for most everyday paperwork, an image signature plus a clear email trail is what people use and it works. The failure mode only appears when someone later disputes the terms — at which point "I have a PDF with a picture of their signature" is weaker evidence than most people assume.
Practical rules for image signatures
If you are going to use one, a few things measurably reduce the risk.
Do not use a high-resolution scan of your real handwritten signature on documents you send widely. It is trivially extractable and reusable. A signature drawn fresh, at screen resolution, is less useful to someone who lifts it.
Flatten after signing. An unflattened signature sits as a separate movable object that can be dragged, deleted or copied out. Flattening merges it into the page. It does not make the document tamper-evident, but it removes the easiest manipulation.
Add a date near the signature as visible text. It proves nothing cryptographically, but it makes the document self-describing and matches how paper documents are read.
Keep the email trail. With image signatures the surrounding evidence is doing most of the work. The message that transmitted the signed file, with its timestamps and headers, is usually more probative than the image itself.
What signing does not do
Signing a PDF does not lock it. Anyone can open a signed document, add pages, change figures and save it. With an image signature nothing will indicate that happened — the signature image sits there looking exactly as valid as before.
This surprises people, and it is the single most important thing to understand. If tamper-evidence is what you need, an image signature does not provide it, no matter how official it looks.
Nor does signing encrypt anything. A signed PDF is fully readable by anyone who has it. If the contents are confidential, that is a separate job — see our guide to password protecting a PDF for what actually secures a file.
Signing in the browser
Our PDF Tools include an e-sign tool that lets you draw a signature and place it on the page, running entirely on your own device. For contracts, that locality is worth something on its own: the document does not get uploaded to anyone's server in order to be signed.
To be clear about what it is: this produces an image-based electronic signature, suitable for the routine cases above. It does not issue certificates or produce cryptographic digital signatures — that requires a certificate authority and key management, which is a different category of service.
Summary
- Image signatures are pictures; they prove little on their own.
- Digital signatures use certificates and detect any modification.
- Routine paperwork is usually fine with the former plus an email trail.
- High-value or disputable agreements deserve the latter.
- Flatten image signatures and avoid distributing a high-resolution copy of your real one.
- Signing is not encryption, and it does not lock the document.
Try it yourself
Our free PDF signing tool needs no account and adds no watermark — use it right now, as many times as you like.
Open PDF Tools →Frequently Asked Questions
Is a drawn signature on a PDF legally binding?
In many jurisdictions an electronic signature can be legally binding, and frameworks like the US ESIGN Act and the EU's eIDAS regulation broadly support that. But binding and provable are different things. A drawn image demonstrates little on its own about who added it or whether the document changed afterwards, which is what matters if the agreement is ever disputed. This is general information, not legal advice — for anything significant, check with a qualified lawyer in your jurisdiction.
What is the difference between an electronic signature and a digital signature?
An electronic signature is the broad legal concept — any electronic mark indicating intent to agree, including a drawn image or a typed name. A digital signature is a specific cryptographic technique using a certificate and private key, which binds the signature to the document's exact contents and reveals any later modification. All digital signatures are electronic signatures; most electronic signatures are not digital signatures.
Can someone copy my signature image from a signed PDF?
Yes. A drawn or uploaded signature is an image embedded in the file, and it can be extracted and reused on another document by anyone with the file. This is the main practical weakness of image-based signing, and a reason to avoid using a high-resolution image of your handwritten signature on documents you distribute widely.
Does flattening a PDF after signing change anything?
For an image-based signature, flattening merges it into the page content, which stops it being moved or deleted as a separate object — a modest improvement. For a cryptographic digital signature, flattening or any other modification invalidates the signature, because the signature covers the exact bytes of the document as signed.