Circle, line, and rectangle objects are not tagged in PDF forms. These objects add no useful information for a user with vision impairment or disabilities. Screen readers will not read any information for these objects.
Do not set the Speak Order Precedence to None for field objects. If you do, Acrobat will cause the screen reader to say “MSAA data prompt not available” when the user tabs into the field, even if the field is read-only.
Subforms are useful for organizing related objects and provide a logical tabbing structure. The default tabbing order is in geographic order, left-to-right, top-to-bottom. If two subforms exist side-by-side, and each subform contains a number of field objects, the tabbing sequence will go through the fields in the first subform before moving on to the next.
When tabbing into a radio button, the screen reader first reads the text for the group object that contains it, and then the On value for the radio button. You should set custom screen reader text for each radio button exclusion group and ensure that the On value for radio buttons matches the caption or is some other meaningful value.
Images may help improve comprehension for users with some types of disabilities, However, many screen readers do not read graphics, which may decrease the accessibility of your form for users with vision impairments. If you choose to use images, provide text descriptions that describe the object and its purpose on the form.
The tabbing order of objects on pages and their master pages is determined by the vertical coordinates of objects. Test your forms to ensure that the screen reader reads objects in the order you want.
Be aware that client-side scripts can interfere with screen readers and keyboards if the script changes the focus of the client application. For example, the change and mouseEnter events, when used with drop-down lists or list boxes, have the potential to cause inappropriate actions. Client-side scripting should be written to avoid problems with screen readers and keyboards. Similarly, avoid scripting events that cause visual effects, such as blinking text, which may increase readability issues for users.
If your form has a large number of objects, tabbing in Acrobat 6.0.2 can be slow. If you are creating an Acrobat 6.0.2-compatible form, adding unnamed subforms around smaller groups of form objects will add levels to the logical structure and fix this problem.
See also 

Design tips for creating accessible PDF forms