To provide an accessible form, you need to understand how screen readers work. You also need to know how to use the LiveCycle Designer ES Accessibility palette to specify information that the screen reader will speak for the objects on forms. Keep in mind these considerations related to implementing accessible forms that can support screen readers:
Introduce the form to tell the users what form is open and how to proceed with its completion. When a form opens, the screen reader will generally read through an entire page, including any text and image objects. When the user begins to tab through the form, the screen reader reads text for each object as it becomes active.
Establish a logical tabbing order that includes all text, fields, buttons, and images to make it easier for users with visual impairment to navigate. Setting the tabbing order is important because PDF screen reader tags are ordered by default from top-to-bottom, left-to-right.
Announce to users any special keystrokes they need to make in order to perform a function. Such keystrokes include actions such as pressing the spacebar to select a button, or the Down Arrow key to select an item from a list box.
In list boxes and drop-down lists, announce the default item selected in the list. Be sure that the user knows to use the Up Arrow and Down Arrow keys to move through the list items. Pressing the Tab key or the Enter or Return key will select the item in the list. Using scripting, you can set the object’s Change event to announce which item is selected from the list.
See also 

Design tips for authoring forms for screen readers