This is because you define your "doc" variable outside of your click event. The first time you click the button the doc variable contains a new jsPDF object. But when you click for a second time, this variable can't be used in the same way anymore. As it is already defined and used the previous time.
change it to:
$(function () {
var specialElementHandlers = {
'#editor': function (element,renderer) {
return true;
}
};
$('#cmd').click(function () {
var doc = new jsPDF();
doc.fromHTML(
$('#target').html(), 15, 15,
{ 'width': 170, 'elementHandlers': specialElementHandlers },
function(){ doc.save('sample-file.pdf'); }
);
});
});
and it will work.