calling code behind function using Javascript

Question

I have a javascript function for ShowDocument in which I need to call a code behind function My Js FUNCTION is as follows:

function showDocument(_id) 
{
    PageMethods.ShowDocument();
}

Codebehind:

[System.Web.Services.WebMethod]
public static void ShowDocument()
{
    byte[] bytes;
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        con.Open();
        using (SqlCommand com = new SqlCommand("SELECT Data FROM FileUploader2", con))
        {
            using (SqlDataReader reader = com.ExecuteReader())
            {
                if (reader.Read())
                {
                    bytes = (byte[])reader["Data"];
                }
            }
        }
    }
}

I have added EnablePageMethods= true in my aspx as well.

But when I debug, it triggers my ShowDocument function but does not go inside my code behind function. It just skips it. What am i doing wrong?