为什么pdf不能完全从输入流下载

Question

我编写此代码以从URL获取和下载PDF并写入OutputStream。但对于某些URL，PDF不能完全下载，下载的PDF只有6KB，我无法在PC上打开此PDF。

String strUrl = (request.getScheme() + "://" + request.getHeader("host") + request.getRequestURI() + "?" + request.getQueryString()).replace("?null", "").replace("%2F", "/");
    URL url = new URL(java.net.URLDecoder.decode(strUrl, "UTF-8"));
    System.out.println("URL ============= " + url);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection(proxy);
    conn.setRequestMethod(request.getMethod());
    conn.connect();
    InputStream is2 = conn.getInputStream();
    System.out.println("----------  is2.available()  ----------- "+is2.available());//NOTE : why Print 2?!!
    OutputStream oos = response.getOutputStream();
    if (url.toString().endsWith(".pdf")) {
        response.setHeader("Content-type", "multipart/form-data");
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename=\"test.pdf\"");
        System.out.println("dl" + rndPdfName + ".pdf");
    }

    byte[] buf = new byte[102400];
    int c = 0;

    while ((c = is2.read(buf, 0, buf.length)) > 0) {
        oos.write(buf, 0, c);
        if (url.toString().endsWith(".pdf")) {
            oos.flush();
        }
        System.out.println("===> buf" + c + " ===> " + buf);//NOTE: just print 2 buf!!!!
    }
    if (url.toString().endsWith(".pdf")) {
        oos.close();
        is2.close();
    }