I will like to add page numbers when i export a pdf ... I just cant get it right

Can i get an example

var cols = [];
cols[0] = {text: 'Left part', alignment: 'left', margin:[20] };
cols[1] = {text: 'Right part', alignment: 'right', margin:[0,0,20] };
var objFooter = {};
objFooter['columns'] = cols;
doc['footer']=objFooter;