JavaScript special effects vs Accessibility

We have lots of PDFs with JavaScript in them, and sometimes the JavaScript is used to alter what appears in the PDF and on the screen.  Example: a dropdown whose choices determine whether or not another field appears, or determines the contents of text field.  

We've been led to believe that this sort of dynamic behaviour violates accessibility.  Is there a way to keep this sort of PDF behaviour, and not violate accessibility? a way to alert the screen reader of the triggered change?

Accessibility has probably become a major issue for just about everybody in the community.  What are your experiences?  lessons learned?