To access Client notes while on a job, make sure that you go to Jobs and select the one that you are working on.

In the Job view, you will see that there are three links beneath the Contact field: JOB, LOCATION, and CLIENT. Clicking on either of these will take you to the notes and documents related to either the Job, the Location or the Client / Contact.

While in there, you can also add more notes by using the + symbol in the bottom right of the app screen.