
Since they are strictly based on asymmetric cryptography, they have a private key, which only the signatory knows, and a public key, which is available to everyone; both are generated through a public key infrastructure algorithm.
In this way, when the signer wants to sign a document, he uses his private key, which is unique and non-transferable, and which is simply in his control; no one & individual can have access to it.
However, to make or temper the signed document, it might become challenging for the attacker as he would have to get the signer’s private key first to falsify the signed information.
Thus, if you're going to use a Digital Signature, you can rest assured and trust it to do any procedure it's complicated for someone to falsify it.