在线预览 Office

1、首先把 Office 转成 PDF，转换方法（http://www.cnblogs.com/zhiqsyr/p/7240432.html）

2、直接向前端返回 PDF 流

        File file = new File(output);    // "/Users/zhiqsyr/Downloads/已注册的手机号170717.pdf"

        resp.setContentType("application/pdf");
        resp.setHeader("Content-Disposition", "inline; filename=" + URLEncoder.encode(file.getName(), "UTF-8"));

        BufferedInputStream br = new BufferedInputStream(new FileInputStream(file));
        byte[] buf = new byte[1024];
        int len = 0;

        OutputStream out = resp.getOutputStream();
        while ((len = br.read(buf)) > 0) out.write(buf, 0, len);

        br.close();
        out.close();

3、前端集成 pdf.js 支持展示