I briefly experimented with something similar last summer. However, in my experiment I used an HTML screenplay, not a PDF document, so I don't know if it will work for you.
http://booksprung.com/how-to-format-...for-the-kindle
The short version: import your HTML screenplay into Celtx (or maybe Final Draft, I dunno), export and clean up any bad tags, swap out the CSS, and then import into Calibre and use that program to convert to a Kindle-friendly .mobi format. Good luck.