How do Chinese citizens get their documents certified?

Since China is not part of the Hague Convention, the process for getting documents certified involves going to a local notary and asking for a “双认证” for Portugal. The work to liaise with the Ministry of Foreign Affairs and the Portuguese consulate should be handled by the notary.

Depending on where you reside, the local notaries may have some variances in fees and requirements. But generally, this should be a straightforward and standard procedure.