How to keep my permanent residence card

It mostly depends on the type of the card but with ANY type of permanent residence you are entitled to the right to stay no matter if you work or not. Though it is important to continue being registered in Belgium.

Yes, exactly! You can even be absent in Belgium for up to a year, and if I am not mistaken, keep you right for permanent residency.
Though if you want to apply for citizenship, your breaks in employment will create problems & your application can be considered ineligible. To remain eligible you will need 5 years of uninterrupted employment.

I am not sure if you speak Dutch but I use usually for this kind info Flemish
https://www.agii.be/ - it was my guide throughout my stay in Belgium.
they provide description for each type of Card and what it means.