<button type="button">
    <a onclick="viewPDF(this,id)" href="" target="_blank"></a>
</button>

function viewPDF(obj,as){
    $.ajax({
        url:"url",
        type:"post",
        dataType:"json",
        data:{
            id:as
        },
        success:function(data){
            if(data.data!==null){
                $(obj).attr("href",data.data)
                window.location.href=data.data;
           }
    
        },
        error:function(){
            console.log("")
        }
    })

html