PDF needs to have a new instance of the PDFSaveOptions created. Look at the JS Reference
PDFSaveOptions
Options for saving a document as an Adobe PDF file, used with the saveAs method. All properties are optional.
Saving to PDF format
// Saves the current document as PDF to dest with specified options
// dest contains the full path and file name to save to
function saveFileToPDF (dest) {
var doc = app.activeDocument;
if ( app.documents.length > 0 ) {
var saveName = new File ( dest );
saveOpts = new PDFSaveOptions();
saveOpts.compatibility = PDFCompatibility.ACROBAT5;
saveOpts.generateThumbnails = true;
saveOpts.preserveEditability = true;
doc.saveAs( saveName, saveOpts );
}
}