Have you considered File.openWithDefaultApplication()?
We switched to AIR one day before due date of project and I'm learning as I go. Forgive the ignorance -- I've ported a six-week project in a few hours, so it's been a very quick study.
This won't work on files in the application directory? It's not working for me -- The PDF is bundled into the AIR app --
In that case, you will have to copy the PDF to a temporary file before opening it.
pdf.copyTo( File.createTempFile() );