Security
What does 'lock the document after signing' do?
Locking freezes the document's form fields at the moment you sign, so nobody can type into them or change what was typed. It does not lock the pages, the text or the ability to sign. Those are separate things.
Under "What can change after signing" in the Digital Signature tool there is a small tick box: "Lock the document after signing". The hint beside it says *"Form fields can no longer be changed once you have signed."* That sentence is exact, and the word that matters is form fields.
What a form field is
A PDF form field is a box you can click and type into inside a PDF reader: a name box, a date box, a tick box, a dropdown. If a document has none of those, locking changes nothing at all, because there is nothing to lock.
You mostly meet them in:
- application forms,
- expense claims and timesheets,
- order forms and delivery notes,
- contracts that come with blanks to complete.
Why you would turn it on
Picture an expense claim. Your colleague fills in "Amount: 200", you check it and sign to approve it.
Without the lock, somebody can open the file afterwards and type 900 into the same box. The signature will notice (a digital signature always notices) but now you are in an argument about what happened rather than in a clear situation.
With the lock on, the box is frozen when you sign. The number you approved is the number that stays.
The same reasoning applies to:
- a form where you are the last person to sign, and nothing more should be filled in,
- a document where the filled-in values are the entire point (amounts, dates, quantities),
- anything you are approving on behalf of someone else.
When to leave it off
Leave it off when other people still have work to do in the document:
- you are the first of three signers, and the others still have fields to complete,
- you are certifying a blank form for others to fill in,
- the document is a template that gets reused.
If you lock too early, the next person finds the boxes dead, and the only fix is to go back to the version before your signature.
What it does not do
This is where people get confused, so plainly:
- It does not stop someone adding a page, a comment, or another signature.
- It does not stop anyone editing the underlying text with a PDF editor.
- It does not password-protect anything.
- It does not replace certifying the document.
It is a narrow, useful tool: form fields, frozen, at your signature.
Lock or certify?
They answer different questions.
| Lock the document | Certify the document | |
|---|---|---|
| What it covers | The form fields only | The whole document |
| Who can use it | Any signer, at any point | Only the first signature |
| Typical use | "Nothing more goes in these boxes" | "I am the author; here is what is allowed" |
| Can be combined? | Yes | Yes |
A common, sensible combination on a finished form: certify at form filling and signing allowed if you are the author handing it out, and lock it when you are the last person to sign it back.
How you know it worked
The result page after signing shows a line: "The document is locked: its form fields can no longer be changed." If that line is not there, the lock was not applied.
Later, anyone opening the file in a PDF reader will find the fields are no longer editable, and Verify PDF Signature will report any attempt to change them as a change after signing.
Common questions
What does locking a PDF after signing actually lock?
The form fields: the boxes people can type into inside a PDF reader. Their contents are frozen at the moment you sign.
Does locking stop someone editing the text of the document?
No. It covers form fields only. Any other change is still possible, but a digital signature makes it detectable.
Should I lock a document that other people still have to sign?
Only if they have no fields left to fill in. Locking freezes the boxes for everybody, so a later signer could find them unusable.
Is locking the same as certifying?
No. Certifying is a statement about the whole document and only the first signature can do it. Locking is about form fields and any signer can apply it. They can be used together.
My document has no form fields. Does locking do anything?
No. With nothing to lock, the option has no effect.
How do I know the lock was applied?
The page you see after signing lists it: 'The document is locked: its form fields can no longer be changed.'