First you have to return an array from the web method. then update it to the dropdown.
PageMethods.updateDrop("select_portfolio", para, "pfname", "pfid", true, "--Select--",'a',false,null,OnSuccess,OnFail);
function OnSuccess(response)
{
ddlpf.options.length=0;
for(i=0;i<response.length;i=i+2)
{
var ddloption=new Option(response[i],response[i+1]);
ddlpf.options.add(ddloption);
}
}
function onFail(){}