思路：通过将需要浏览的pdf通过后台转为文件流传给前端，来解决跨域问题。

js代码写在viewer.html里，放到引用pdf.js的位置前面

js代码,来接收文件流：

<script>
      var PDFData = "";
      $.ajax({
          type:"get",
          crossDomain:true,
          async:false, // false是不采用异步的方式
          mimeType:'text/plain;charset=x-user-defined',
          url:"http://localhost:8080/Convert?type=getPdf",
          success:function (data) {
              PDFData = data; // data是byte[]数组
          }

      });
      var rawLength = PDFData.length;
      // 转换成pdf.js能直接解析的Uint8Array类型，见pdf.js-4068
      var array = new Uint8Array(new ArrayBuffer(rawLength));
      for(var i = 0;i<rawLength;i++){
          array[i] = PDFData.charCodeAt(i)&0xff;
      }
      var pdf_url = array;

    </script>

后台，我写通过java了一个servlet来把文件转成文件流：

package main;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

@WebServlet(name = "Convert")
public class Convert extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("application/pdf");
        response.setHeader("Access-Control-Allow-Origin", "*"); // 解决请求头跨域问题
        ServletOutputStream sos = response.getOutputStream();
        String destUrl = "http://39.107.117.248/pdf-store/caseFile.pdf";
        URL url = new URL(destUrl);
        HttpURLConnection httpUrl = (HttpURLConnection)url.openConnection();
        // 连接指定的网络资源
        httpUrl.connect();
        // 获取网络输入流
        BufferedInputStream bis = new BufferedInputStream(httpUrl.getInputStream());
        int b;
        while ((b = bis.read())!= -1){
            sos.write(b);
        }
        sos.close(); // 这里有点和c语言里的读取文件有点像
        bis.close();
    }
}

参考链接：https://blog.csdn.net/anciend/article/details/80062213，感谢Anciend的分享