When the capital ship of the necro virus is destroyed by PDF fire ,is the "steal technology" still used?

You've asked this twice now. The answer is no. The race sheet for Nekro is explicit: "except PDS".

That exept that made us confused..

Exept , you win the tech or except you dont ?

I think he is asking if the Necro Flagship system wide destruction triggers his racial trait if the Flagship is sank due to PDS fire. I would say no because PDS fire is during step 3 of a tactical action, not the Space Battle step, step 4, of a Tactical action. So technically the Necro Virus did not destroy an enemy unit during a Space Battle or Invasion Combat.

The "except" means that they steal techs if they kill a unit in any battle BUT NOT with PDS fire. This means that the Nekro will not steal a tech if THEY shoot down an enemy with PDS fire. And if the Flagship is destroyed by PDS fire, which happens outside battle, the resulting blast will NOT allow the Nekro to steal a tech.