What chooses the page setup in this script

Not entirely sure what you mean by a button that runs a script in the background.

Here's a script that creates a new document after first asking the user to select a document preset (You have to create the presets in advance, of course). If you save it as a .jsx file in your scripts folder, you can run it from the Scripts Panel. Kinda like a button. There are probably a zillion examples of this kind of thing on the internet.

If '"in the background" means you want something that happens automagically, then you probably want an event listener, one that fires on the "beforeNew" event.

Bob (still angling for that "correct answer" tag; Uwe's got plenty )

var presetNames = app.documentPresets.everyItem().name;

var userSelected = chooseFromList("Choose a Document Preset",presetNames,-1);

var myDocument = app.documents.add(true,app.documentPresets.item(userSelected));

function chooseFromList(windowName,theList,defaultItem) {

var win = new Window("dialog",windowName);

var myGroup = win.add("group");

myListBox = myGroup.add ("listbox",undefined,theList);

myListBox.selection = defaultItem;

var listboxDepth = theList.length * 25;

if (listboxDepth > 800) {

listboxDepth = 800;

} //end if

myListBox.preferredSize = [250,listboxDepth];

win.myButtonsGroup = win.add ("group");

win.myButtonsGroup.orientation="row";

win.myButtonsGroup.buttonOK = win.myButtonsGroup.add ("button",undefined, "OK");

if (defaultItem < 0) {  

win.myButtonsGroup.buttonOK.enabled = false;

} // end if

win.myButtonsGroup.buttonCancel = win.myButtonsGroup.add ("button",undefined, "Cancel");

//OK button will be disabled until user selects an item in the listbox

myListBox.onChange = function () {

win.myButtonsGroup.buttonOK.enabled = true;

}

var userClicked = win.show();

if (userClicked == 1) {

var userSelected = myListBox.selection.text;

}

else {

var userSelected = undefined;

}

return userSelected;

} // end chooseFromList