Short answer. No. A waiver of your legitime signed while your parents are still living is void under the Civil Code, no matter how formal the paper looks. When they die you may still claim your legitime — but whatever you were given for signing must be brought back into the computation.
What the law says
Every renunciation or compromise as regards a future legitime between the person owing it and his compulsory heirs is void, and the latter may claim the same upon the death of the former
Civil Code, Article 905 — Renunciation Of Future Legitime. Read the full provision →
Why the document fails even if you signed it willingly
The law does not treat this as a bargain between adults that the courts will simply enforce. A future legitime is not yet anyone's property — nobody has died, so there is no estate to give up. What the Civil Code voids is precisely the attempt to settle that share in advance between the parent who owes it and the child who will one day be entitled to it. Void means it never had legal effect at all, so it is not something you can later ratify by silence, by cashing the money, or by letting years pass. Notarisation does not cure it. A lawyer's signature as witness does not cure it. Calling the paper a deed of quitclaim, a compromise agreement or a family settlement does not cure it either, because the law looks at what the document does, not what it is titled.
What you must give back when you finally claim
The rule is not a windfall. The same provision that lets you claim the legitime also requires that you bring to collation whatever you received by virtue of the renunciation or compromise. In practice this means the cash, land or business share handed to you when you signed is counted as an advance against your inheritance and deducted from what you eventually receive. So a child who took a large sum to sign a waiver may end up with little or nothing further, without the waiver ever being valid. Keep records of what you actually received — receipts, bank transfers, transfer certificates of title — because collation is an accounting exercise, and a vague claim that nothing was given is hard to sustain against documents.
What this rule does not cover
This protection is limited to the legitime, the portion the law reserves for compulsory heirs such as children, the surviving spouse and, in some situations, parents. It does not stop your parents from disposing of the free portion of their estate during their lifetime, nor does it stop them from making genuine donations or sales to anyone they choose while living. It also does not apply once the parent has died: an heir may then validly repudiate an inheritance that has already opened, though that is a different act with its own formal requirements. If you are being pressured to sign now, the safer step is to decline and take the document to counsel before any money changes hands.