Once a PDF has been digitally signed, it is usually impossible to edit without compromising the signature. And it’s precisely the purpose of an electronic signature not to be able to tamper with a signed document! However, in some cases, it is possible to amend the file in a way that keeps the original signature valid.

Is it possible to modify a signed PDF?

Legal aspects

From a legal point of view, a signed document will not be valid if there is any evidence that it was altered after signing.
Most of the time, it’s not even possible to modify a signed PDF because the content will be locked, and the user will get a warning message.
However, sometimes, it can be helpful to modify typos or add notes to a signed document without compromising the initial signature and asking the signer to review the entire document again. It is especially true when signing long contracts, for instance.
Of course, we’re not talking about trying to modify the content of a signed document at length. People will always use tricks and software loopholes to achieve this, but it can be relatively easy to prove that the document has been tampered with if necessary. And generally speaking, if the content of a document needs to be significantly edited, people will need to resign it.

The two sides of a valid signature

An industry-valid signature

Depending on your industry, country, and line of business, there are different ways to sign a PDF.

In the European Union, the eIDAS regulation allows three types of electronic signatures:

  • the simple electronic signature suitable for everyday acts or those with low legal or financial consequences for the signatory.
  • the advanced electronic signature, which includes the use of a digital certificate.
  • the qualified electronic signature, generally used for authentic acts of notaries, lawyers, bailiffs, courts, and the like. Its security criteria are comparable to those of the advanced digital signature.

It is up to the entity asking for a signature to implement the right signing protocol and method.
For example, in most countries, you cannot use a simple electronic signature when selling a house. If you do so, the act will be considered invalid because, legally, the notary should have used a qualified electronic signature.