Sistema de firmas ridiculo

Seems to me like your tutor and yourself are not familiarized with the signing workflow needed.

Either the PDF file would be signed using electronic signatures with the "Fill & Sign" tool, or the users will employ certificate-based digital signature.

To use self-signed Certificate-based digital signatures you must add a digital signature field using the "Certificate" tool.

These are two different type of signing workflows.

If you were the creator of this PDF you need to establish which one. Moreover, leaving the PDF opened like that allows anyone to sign with other PDF viweing tools.

I would suggest to employ the use of Adobe Sign, in which case, when you create the PDF it allows you to add and assign signing blocks for a specific user based on their email address as the main authentication factor plus adding a password that you create only for that

intended user to open  the PDF.

Only the intended recipient will be allowed to fill and sign the intended block(s), nobody else.

Aan added benefit of employing Adobe Sign is that your signing recepients are not required to purchase Adobe Acrobat Pro DC nor have installed in their system Adobe Reader.

They can sign via web browser, and even through a mobile device.

You'll also be able to track the signing process while the file is in-transit, and modify the agreement if needed.

You can use the Fill & Sign toolbar and sign the page anywhere you want. If the author of the form wanted you to use a digital signature they should have added a signature field to the file.