string siteURL = ConfigurationManager.AppSettings["siteURL"];
        string userName = ConfigurationManager.AppSettings["userName"];
        string password = ConfigurationManager.AppSettings["password"];

        ClientContext clientContext = new ClientContext(siteURL);
        SecureString securePassword = new SecureString();
        foreach (char c in password.ToCharArray()) securePassword.AppendChar(c);
        clientContext.Credentials = new SharePointOnlineCredentials(userName, securePassword);

        Web web = clientContext.Web;
        ListCollection listCollection = clientContext.Web.Lists;
        clientContext.Load(
            listCollection,
            lists => lists
                .Include(
                    list => list.Title,
                    list => list.Hidden,
                    list => list.RootFolder.Files)
                .Where(list => !list.Hidden)
             );
        clientContext.ExecuteQuery();
        FileCollection files = listCollection.Where(x => x.Title == "Documents").FirstOrDefault().RootFolder.Files;
        File file = files.Where(x => x.Name == "Excel_File.xlsx").FirstOrDefault();