Almost everyone who becomes a fiscal resident in Portugal will pay only 10% taxes on their pension. But there are certain exceptions such as citizens with residency in tax havens or countries like Sweden which withdrew from the tax convention with Portugal.
From the moment you become a Portuguese tax resident, it is up to the Portuguese government to tax your pension. For the first 10 years the Portuguese government will tax you only 10% on your pension income. The same applies to your dividends, capital gains taxes, royalties and real estate rent income. You are exempt from inheritance taxes.