Interactive buttons show with a square box around them...

Hi,

I've created an interactive pdf using InDesign. I use buttons that show fine in the pdf but when upload it and previewing in the browser, various browsers, the buttons show a square shape around them. How can I fix this?

Here is how the document looks like in pdf:

Here is how it looks like when uploading to the server and previewing in the browser:

These buttons show/hide other buttons in the page, no animation or effects have been applied.

Any idea why is this happening?

Thanks