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You can make a circle without background with Illustrator and save it as PDF. Then import it as icon in a button Form field. --> Show/Hide the button.
See a sample file (red circles on pictures) at :
http://www.abracadabrapdf.net/download/fr/forms/rollOver.pdf
To change your subscription: http://www.pdfzone.com/discussions/lists-pdfforms.html
