Diesel,
The JKI booklet is the job knowledge interview booklet. If you get invited to a stage 1 interview, they will send you this booklet via email approximately 2 weeks prior the interview. It is 60 pages of flying basics.
PPL = Private Pilots License. The information in the JKI booklet is at about this level of understanding.
The medical form is fairly basic and not at all in depth. From what i have read, the really in-depth stuff is to be expected in HK where you undergo a comprehensive medical assessment.
If you are serious about wanting to become a cadet, i suggest you read this thread from the absolute beginning. It will take a few days, but will answer your questions. For anyone else interested, don't take the comments of the detractors in this thread too seriously. Take the time to do some research of your own and you'll find that while the cadetship isn't the greatest offer going, it certainly isn't the worst either. Having spoken with a few existing S/O's who have been through the cadetship (on worse T's&C's), they all agree that it is quite manageable, so long as you don't have high expectations.
Just make sure you know what you are getting yourselves into.
Best of luck.