protected void Page_Load(object sender, EventArgs e)
{

    var spContext = SharePointContextProvider.Current.GetSharePointContext(Context);

    int listItemID;

    listItemID = GetListItemIDFromQueryParameter();

    using (var clientContext = spContext.CreateUserClientContextForSPHost())
    {
        clientContext.Load(clientContext.Web,
        web => web.Title,
        web => web.CurrentUser,
        web => web.Lists);

        List doclist = clientContext.Web.Lists.GetByTitle("Documents");
        Microsoft.SharePoint.Client.ListItem item = doclist.GetItemById(listItemID);

        string title = item.File.Title;
    }
}