There are all sorts of medical reports you can use to help with your application for PIP, if you are seeing any specialist then when they write to your doctor this is a report, you can ask them to send you a copy.
If you get referred to ADHD for an assessment then ask for a copy of the assessment once it is done, I would advise you to read the report carefully before you send it in as some reports to not always help your application.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems