C# MDI问题

vs2008 、win7
vs2008写了个 浏览 “.pdf”的程序 如图
http://b64.photo.store.qq.com/http_imgload.cgi?/rurl4_b=c023a75f6a0c34796fa20c45de0040fb5ca4a4f1efb4fdfb32dd86ee9a77e340f72d0bfb44e2c4ee4e7a551cbdb8757a36ea19561a17121025f4aeeabf0b2ff39262d67c6d688cb40882fd0fef2f54683065c8da&a=63&b=64然后把这个程 放入 MDI 中 “.pdf”文件内容却不显示了 如图
http://b60.photo.store.qq.com/http_imgload.cgi?/rurl4_b=c023a75f6a0c34796fa20c45de0040fbcc197bd53e3a47a5660ec1dff6d9e5905948c6f896a55bf92d28a4f93028fd9c87e24fbcf4e0eb08a782f55ab55384338009954f2887438b9c51b5c8cc478e24974929a9&a=59&b=60 这个是为什么呀 ？希望高手们解决下  谢谢了呀！