iframe.contentWindow.PDFViewerApplication.pagesCount
iframe.contentWindow.PDFViewerApplication.page

// 注： 以下形式也可以跳转到指定页
// https://commoncms.ynyplt.cn/file-server/13648711629/9b613676-d867-47e3-873d-47d6d59015d5.pdf#page=1

// 接口部分
@RequestMapping("/pdfdownload")
@ResponseBody
public void fileDownload(@RequestParam("filename") String filename,
                            HttpServletResponse response) throws UnsupportedEncodingException {
    // 通过网络获取文件流
    DownloadUtils.getFileStream(filename, response);
}

package com.example.mybatis.utils;

import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.net.ConnectException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

public class DownloadUtils {
    // 下载工具部分
// 将文件流传送到response
    public static void getFileStream(String filename, HttpServletResponse response) {
        try {
            // 根据文件url下载文件
            openFile(filename, response);
        } catch (Exception e) {
            System.out.println("下载失败");
        }
    }

    // 通过url下载文件的方法
    private static void openFile(String filePath, HttpServletResponse response) {
        // 服务器状态
        int HttpResult;
        try {
            // URL
            URL url = new URL(filePath);
            // URL连接对象
            URLConnection urlconn = url.openConnection();
            urlconn.connect();
            HttpURLConnection httpconn = (HttpURLConnection) urlconn;
            HttpResult = httpconn.getResponseCode();
            if (HttpResult != HttpURLConnection.HTTP_OK) {
                System.out.println("网络连接失败");
                throw new ConnectException("网络连接失败");
            } else {
                InputStream in = urlconn.getInputStream();
                try {
                    String fileName = "test.pdf";
                    response.setContentType("application/pdf;charset=utf-8");
                    response.addHeader("Content-Disposition", "attachment;filename=\"" + fileName + "\"");
                    int b = 0;
                    byte[] buffer = new byte[512];
                    while (b != -1) {
                        b = in.read(buffer);
                        if (b != -1) {
                            response.getOutputStream().write(buffer, 0, b);
                        }
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                    System.out.println("文件取得失败");
                } finally {
                    try {
                        if (in != null) {
                            in.close();
                        }
                        response.getOutputStream().flush();
                    } catch (IOException e) {
                        e.printStackTrace();
                        System.out.println("文件取得失败");
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("文件取得失败");
        }
    }
}