I tried the below code for that, but it adds pagedown buttons to only the first .wmd-input.

if ($(".wmd-input").length > 0) {
    var converter = new Markdown.Converter();
    var help = function () { alert("Do you need help?"); }
    var options = {
        helpButton: { handler: help },
        strings: {quoteexample: "whatever you're quoting, put it right here"}
    };
    var editors = [];
    var i = 0;
    $(".wmd-input").each(function() {
        editors[i] = new Markdown.Editor(converter, "", options);
        editors[i].run();
        i = i + 1;
    });
}
 
                        
Looks like i have to add unique ID for each element of wmd. I mean
wmd-input,wmd-previewandwmd-button-bar. I modified this id attributes programmatically. This can be done with modifying manually but my length of inputs are dynamic.So when this ID attributes is set, i called the editor with postfix variable and problem solved.