When designing accessible forms, you can define text that is read by screen readers. You must create a tagged PDF form so that the screen reader can read the text. You do this by generating accessibility tags when saving the form design as a PDF file.
In LiveCycle Designer ES, the default behavior is to create tagged PDF forms.
The tagging applied for accessibility increases the file size, which reduces performance. To help offset the effects on performance for accessible forms, choose design elements carefully.
See also 

Performance considerations for accessible forms