PFAS are many different molecules.

For example PTFE is a large molecule with strong bonds, and as a consequence isn't very reactive and likely safe.

On the other hand, perfluoroalkyls such as PFOA have the same shape as fatty acids, so they bind to the same places such as in the liver, which makes them grave health hazards.

Many precursors used for making PFAS are also toxic, so for example, even if PTFE is safe, manufacturing it isn't.