In cases the signer is unable to sign his own name, there are the signature methods SMS Code and Code via E-mail.
In this case, the document must have two witnesses present to assist the illiterate person in the signing process, and the witnesses will also need to sign, as recquiered by law (see your country's legislation if you are not brazilian).
You can also ask signers for a Selfie to add another layer of security to your document.
Learn more about it here.