var w = new Window("dialog", "Choose a Preset");
var dropdownlist = w.add("dropdownlist", undefined, app.PDFPresetsList);
    dropdownlist.preferredSize.width = 200; 
    dropdownlist.selection = 0;
    var choice = dropdownlist.selection;
    dropdownlist.onChange = function () {
        choice = dropdownlist.selection;
    };
var button = w.add("button", undefined, "OK");
    button.onClick = function () {
        saveDocsAsPDF(choice);
        w.close();
    };
w.show();

function saveDocsAsPDF(choice) {
// original SaveDocsAsPDF script
    try {
        if (app.documents.length > 0 ) {
            var destFolder = null;
            destFolder = Folder.selectDialog('Select folder for PDF files.', '~');
            if (destFolder != null) {
                var options, i, sourceDoc, targetFile;	
                options = getOptions();
                for ( i = 0; i < app.documents.length; i++ ) {
                    sourceDoc = app.documents[i];
                    targetFile = getTargetFile(sourceDoc.name, '.pdf', destFolder);
                    sourceDoc.saveAs( targetFile, options );
                }
                alert('Documents saved as PDF');
            }
        } else {
            throw new Error('There are no document open!');
        }
    } catch(e) {
        alert( e.message, "Script Alert", true);
    }
    function getOptions() {
        var options = new PDFSaveOptions();
        options.pDFPreset = choice;
        return options;
    }
    function getTargetFile(docName, ext, destFolder) {
        var newName = "";
        if (docName.indexOf('.') < 0) {
            newName = docName + ext;
        } else {
            var dot = docName.lastIndexOf('.');
            newName += docName.substring(0, dot);
            newName += ext;
        }
        var myFile = new File( destFolder + '/' + newName );
        if (myFile.open("w")) {
            myFile.close();
        }
        else {
            throw new Error('Access is denied');
        }
        return myFile;
    }
}