People with ADHD can still focus, it's just that their brain can't self-regulate their focus like in a neurotypical person. They may hyperfocus on trivial things and not focus as much (or at all) on more important things.
I think Peter shows signs of hyperfocusing when working on his science, and in the way he tends to get lost in his head sometimes and not notice a person talking to him (I remember this happening multiple times in the Lee/Ditko run and at least once in the JMS run).
Since we are on this topic, I wrote a more detailed post about it in another thread.
The only thing I would add is that JMS also depicted him as someone who has trouble sleeping at night due to an active mind, which is another ADHD symptom.