Indications are that Portugal will be announcing how it will deal with UK citizens in the event of "no deal" Brexit very shortly.
Information coming from the UK embassy staff suggests that those with permanent residence documentation need to take no further action. But those who are not formally registered, or who have been present for 5 years (and who did register) but who have not obtained a permanent residence card should do so as soon as possible.
Please see my Brexit page for further information.