Digital signatures are like paper-based signatures, but better because they include encrypted information so you can verify their authenticity.
You can digitally sign a document to securely record the identity of the author, and to verify the content has not changed. Digital signatures in a PDF document can show text, or an image such as a photo or an image of your handwritten signature.
Once the final digital signature has been added to a document, the lock function will be presented.
Locking a document disables all tools that can be used to change the document, such as editing, commenting, form filling, and signing.
To unlock a document, the final signer must clear their signature by right-clicking on the signature and selecting 'clear signature field.'
If you wish to lock a document without a digital signature you can use Nitro's Certify feature - read the Certify PDF files article.