Stay safe, assume everything is illegal and what's not requires a license and money.
This page is only a draft
This page will try to translate what portuguese law says, a translation to portuguese is also needed, but due to it's complexity becomes irrelevant.
If you are rich you already know you don't need this and your legal team must already inform you how to avoid everything.
Check gang mafia invoice law.
Documents to be translated / summarized;
1 - License to work
You will need a license to work, they call it a registry. Visit the web page portaldasfinancas.gov.pt you should have an address, a phone and contribution ID they call NIF.
They have two kinds of categories, "simple" if you make less than 10k a year (below poverty line in EU) and a "complex" if you have normal degree of success in the business.
Once you have the license you risk fines or even prison if you fail something, if you don't profit for what I understood until now you will have to pay 20 euro per month anyway.