Java Script changes my form to FDF, how do I send as PDF?

I have a java script that I use to submit a form to 2 different addresses depending on a certain field.

But when it send the form it sends as an FDF not a PDF; I need it sent as a PDF.

How do I ensure it is sent as a PDF not an FDF?

I am an absolute novice at java script so any help would be appreciated.

thanks in advance

R.Keev

var addr= "";

if (this.getfield("DropDown1").value=="Yes")

{

addr="1stmaillist@whatever";

}

else

{

addr="2ndmaillist@whatever";

}

This.submitform("Mailto:" + addr);