httpurlconnection下载pdf文件打不开的原因，和解决代码

//之前使用reader读取返回内容
BufferedReader reader = new BufferedReader(new InputStreamReader(httpConn.getInputStream(),"UTF-8"));

//修改为input读取返回内容
BufferedInputStream bin = new BufferedInputStream(httpURLConnection.getInputStream());

package com.myitext.test;

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * Httpurlconnection下载pdf文件
 * @author Admin
 *
 */
public class HttpurlconnectionTest {
    
           public void downPdf(String urlPath,String content) {
                try {
                    URL url=new URL(urlPath);
                    HttpURLConnection con=(HttpURLConnection)url.openConnection();
                     
                    //设置参数
                    con.setDoOutput(true);     //需要输出 POST请求
                    con.setDoInput(true);      //需要输入
                    con.setUseCaches(false);   //不允许缓存
                    con.setRequestMethod("POST");      //设置POST方式连接
                     
                    //设置请求属性
                    con.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
                    con.setRequestProperty("Connection", "Keep-Alive");
                    con.setRequestProperty("Charset", "UTF-8");
                  　con.connect();
                         
                    //建立输入流，向指向的URL传入参数 POST请求
                    DataOutputStream dos=new DataOutputStream(con.getOutputStream());
                    dos.writeBytes(content);
                    dos.flush();
                    dos.close();
                     
                    //获得响应状态
                    int resultCode=con.getResponseCode();
                    if(HttpURLConnection.HTTP_OK==resultCode){
　　　　　　　　　　　　　　//代码修改的主要地方           
                        BufferedInputStream bin = new BufferedInputStream(con.getInputStream());
                        File file = new File("D:/pdf/target.pdf");
                        if (!file.getParentFile().exists()) {
                            file.getParentFile().mkdirs();
                        }
                        OutputStream out = new FileOutputStream(file);
　　　　　　　　　　　　　　//如果是reader,size就是String类型了，while循环也要相应的修改
                        int size = 0;
                        byte[] buf = new byte[1024];
                        while ((size = bin.read(buf)) != -1) {
                            out.write(buf, 0, size);
                        }
                        bin.close();
                        out.close();
                    } 
                } catch (Exception e) {
                    
                    e.printStackTrace();
                }
          }
}