Answer
Falsification of a public, official, or commercial document is punished more severely because such documents carry a presumption of regularity and public faith. Under Article 172 of the Revised Penal Code, a private individual who falsifies a public, official, or commercial document (or a public officer who does so without taking advantage of his position) is liable, and no proof of damage or intent to cause damage is required — the undermining of public faith itself is the injury.
Falsification of a private document, by contrast, requires that the offender caused damage to a third party or at least acted with intent to cause such damage. This element of damage is the distinguishing feature: harm to a private interest must be shown for private-document falsification, whereas for public or commercial documents the violation of public faith is enough. The penalties and elements are set out in Articles 171 and 172.