You drop your signature onto the page and the viewer rejects it. Or the signature tool is greyed out entirely. The PDF was sent specifically for you to sign — and you can't.
The most common cause is restrictive permissions, not anything wrong with you or your signature.
What's actually going wrong
Signature permissions disabled. The PDF was created with annotation and signing turned off — common with PDFs exported from older tools or government forms. File already has a finalised signature. Adding more signatures isn't possible after the file is sealed. Wrong type of signature expected. The PDF expects a digital certificate signature, not a drawn or typed one.
Check if you can add a comment or any annotation. If nothing works, the file is locked broadly, not specifically to signing.
The quick fix
Use Flint's sign tool. Flint accepts drawn, typed, and uploaded signatures, and bakes them into the page as flat content — which works on documents that block traditional signature layers.
If the PDF expects a specific signature format you can't provide, ask the sender for the requirements or an alternative way to sign (e.g., scanned signed pages).
If that didn't work
For PDFs that are signed and sealed, you can't add to them. Get an unsigned copy from the sender, sign that, and they can collect signatures from there.
For PDFs with locked permissions you legitimately need to sign, unlock the PDF if you have the owner password, then sign. If you don't have the password, contact the sender — they made the file and they can send an editable version.
Prevent it next time
When sending PDFs for signature, don't apply permission locks. Use Flint's flow which keeps the file accessible to signers. And when expecting to sign a document, ask the sender if they used signature-friendly export settings before they send.
FAQ
Why does my PDF say 'this document has been signed and cannot be modified'?
Someone applied a final signature that seals the file. No additional signatures or edits possible. Ask the sender for a fresh copy without the final signature.
Can I sign a password-protected PDF?
Only after unlocking it. Use Flint's unlock-pdf tool with the owner password, sign, then re-apply protection if needed.
What's the difference between digital and electronic signatures?
Electronic signatures are drawn, typed, or uploaded marks. Digital signatures use cryptographic certificates. Both are legally valid for most uses; some institutions require certificates specifically. Flint supports both.
Why is my signature appearing tiny on the PDF?
The signature image is being scaled to a small target area. Resize before placing, or drag the placed signature to the size you want. Flint's signing tool lets you resize after placement.
If a PDF blocks signing, Flint's sign tool usually gets through anyway. For files that are truly sealed, request a fresh copy.