检测pdf文档是否加密

using System;
using System.Text;
using System.IO;
using System.Net;
using System.Web;
namespace APIStore
{
    class HttpRequest
    {
        /*
         * HTTP的Post请求方式(推荐)
         * strUrl 请求地址
         * param 请求数据
         */
        public string requestPost(string strUrl, string param)
        {
            HttpWebRequest httpWebRequest = WebRequest.Create(strUrl) as HttpWebRequest;
            httpWebRequest.Method = "POST";      //指定允许数据发送的请求的一个协议方法
            httpWebRequest.ContentType = "application/x-www-form-urlencoded";       //设置 ContentType 属性设置为适当的值
            httpWebRequest.Headers.Add("appId","你的appId");
            httpWebRequest.Headers.Add("appKey","你的appKey");
            httpWebRequest.Headers.Add("version","5.2.3");
            byte[] data = Encoding.UTF8.GetBytes(param);
            using (Stream stream = httpWebRequest.GetRequestStream())
            {
                stream.Write(data, 0, data.Length);     //写入数据
            }
            WebResponse webResponse = httpWebRequest.GetResponse() as HttpWebResponse;        //发起请求,得到返回对象
            Stream dataStream = webResponse.GetResponseStream();
            StreamReader reader = new StreamReader(dataStream, Encoding.UTF8);
            string returnStr = reader.ReadToEnd();
            // Clean up the streams and the response.
            reader.Close();
            webResponse.Close();
            return returnStr;
        }
        /**
        * 获取内容
        */
        public void getContent()
        {
            string url="https://api-v2.1dq.com/v5/Tool/checkFileEncryption.json"
            string param="file_id=signed-30bfb87dd63a4c7a5f720dbbbd7c2b2b-9808d27b-0004"
            string returnStr = null;
            returnStr = "post result:" + this.requestPost(url, param);
            Console.WriteLine(returnStr);
        }
    }
}