It would appear it is making the PDF accessible for the disabled. This is what you have to do:
In order to meet the needs of the disabled, all forms should have the following capabilities:
A screen reader should have access to the value and `prompt' for each item, so that a blind or low vision user can hear it. In this paper that is referred to as the 'speakable prompt'.
All functionality should be accessible from the keyboard only, so that a user does not have to manipulate a mouse or other pointer device.
All color usage should be under the control of the user, so that a low vision or color blind user can see all of the content.
There should be no reliance on timed functions, so that a user does not need to respond in a set amount of time.
Flashing or animations should be avoided, or if present allow a mode that disables them.