Flash - Opening PDF Document With Adobe Acrobat Reader?

I want to open up a pdf-document (included within the *.air-package) with adobe acrobat (or any other pdf-reader). I've followed this solution openWithDefaultApplication fails on files in application folder.

var _myfilename = "John_Doe-tax_return_2011.pdf";
// = my filename
//run:
var realFile:File = File.applicationDirectory.resolvePath(_myfilename);
var tempFile:File = File.createTempFile();
realFile.copyTo(tempFile,true);
tempFile.openWithDefaultApplication(); //nothing happens ?
//end;

I've tested it all, so it's not a directory issue
trace(tempFile.extension)
// > tmp (?? tempFile.extension = "pdf" doesn't works)
trace(realFile.exists)
// > true (original pdf-document exists!)