What is the error message? What file are you trying to export? A reponse as PDF, the form as PDF, the design of the form, an attachment...?
I was trying to export the design of the form, I also tried to export it as a PDF. It was a customer feed back form.
It would help me figure out your issue if you can send me the error message you get when you try to export.
What browser are you using? Are you on a mac or a windows machine?
You might want to try a different browser and see if you get the same result.
What is your Flash Player version? You can find it if you go on this website :