该例实现的是后端返回的base64的字符串，里面包含图片和文字数据，保存为pdf到手机内存。
1、添加权限

   <!--    在cd卡中创建和删除的权限·-->
   <uses-permission android:name="permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
    <!--    读外存-->
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <!--用于写入缓存数据到扩展存储卡，需经动态申请-->
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

2、在application中添加可分区存储的属性

        android:requestLegacyExternalStorage="true"

3、动态申请写入数据到存储卡的权限

    /**
     * 检查是否享有写入外存的权限
     */
    private void StartCheckWriteExternalPermission() {
        if (PermissionChecker.checkSelfPermission(JSBridge.getActivity(), Manifest.permission.WRITE_EXTERNAL_STORAGE) != PermissionChecker.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(JSBridge.getActivity(), new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, SAVE_PDF_EXTER_PERMISSION);
        } else {//已经有权限
            startDowloadPdf();//开始下载
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        if (requestCode == SAVE_PDF_EXTER_PERMISSION) {//存外存权限
            int length = Math.min(permissions.length, grantResults.length);
            for (int i = 0; i < length; i++) {
                if (TextUtils.equals(permissions[i], Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
                    if (grantResults[i] != PackageManager.PERMISSION_GRANTED) {
                        Toast.makeText(JSBridge.getActivity(), "下载失败，请先前往设置打开存储权限", Toast.LENGTH_SHORT).show();
                    } else {
                        startDowloadPdf();//开始下载
                    }
                    break;
                }
            }
        }

4、下载代码

    /**
     * 下载pdf
     */
    private void startDowloadPdf() {
        JSONObject returnBody=new JSONObject();
        String fileBase64=savePdfJson.getString("fileBase64");
        BufferedInputStream bis=null;
        FileOutputStream fos=null;
        BufferedOutputStream bos=null;
        try {
            fileBase64=fileBase64.replaceAll("^data:[a-zA-Z]+/[a-zA-Z]+;base64,", "");//此部分因传过来的数据有前缀而写，可忽略
            byte[] bytes=Base64Util.decode(fileBase64);//将给定的Base64编码的字符串解码为一个新的字节数组，此为自己实现的工具类方法
            ByteArrayInputStream byteArrayInputStream=new ByteArrayInputStream(bytes);
            bis=new BufferedInputStream(byteArrayInputStream);
            Date date=new Date();
            SimpleDateFormat dateFormat = new SimpleDateFormat("yyyyMMddhhmmss");
            String currentData= dateFormat.format(date);
            path1=Environment.getExternalStorageDirectory().getPath()+"/"+currentData;
            File file=new File(path1+"/"+"xx.pdf");
            File path = file.getParentFile();
            if (!path.exists()) {
                path.mkdirs();//创建多层目录
            }
            fos = new FileOutputStream(file);
            bos = new BufferedOutputStream(fos);
            byte[] buffer = new byte[1024];
            int length = bis.read(buffer);
            while (length != -1) {
                bos.write(buffer, 0, length);
                length = bis.read(buffer);
            }
            bos.flush();
            Toast.makeText(JSBridge.getActivity(), "保存成功", Toast.LENGTH_SHORT).show();
        } catch (IOException e) {//失败
            Toast.makeText(JSBridge.getActivity(), "保存失败", Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        } finally {
            try {
                bis.close();
                bos.close();
                fos.close();
            } catch (IOException e) {//下载失败
                Toast.makeText(JSBridge.getActivity(),"保存失败", Toast.LENGTH_SHORT).show();
                e.printStackTrace();            
            }
        }
    }

完结。