//工具类
    public static class FormUpload
    {
        private static readonly Encoding encoding = Encoding.UTF8;
        public static HttpWebResponse MultipartFormDataPost(string postUrl, string userAgent, Dictionary<string, object> postParameters)
        {
            string formDataBoundary = String.Format("----------{0:N}", Guid.NewGuid());
            string contentType = "multipart/form-data; boundary=" + formDataBoundary;
            byte[] formData = GetMultipartFormData(postParameters, formDataBoundary);
            return PostForm(postUrl, userAgent, contentType, formData);
        }

        private static HttpWebResponse PostForm(string postUrl, string userAgent, string contentType, byte[] formData)
        {
            HttpWebRequest request = WebRequest.Create(postUrl) as HttpWebRequest;
            if (request == null)
            {
                throw new NullReferenceException("request is not a http request");
            }
            request.Method = "POST";
            request.ContentType = contentType;
            request.UserAgent = userAgent;
            request.CookieContainer = new CookieContainer();
            request.ContentLength = formData.Length;
            using (Stream requestStream = request.GetRequestStream())
            {
                requestStream.Write(formData, 0, formData.Length);
                requestStream.Close();
            }
            HttpWebResponse res;
            try
            {
                res = (HttpWebResponse)request.GetResponse();
            }
            catch (WebException ex)
            {
                res = (HttpWebResponse)ex.Response;
            }

            return res;

        }

        private static byte[] GetMultipartFormData(Dictionary<string, object> postParameters, string boundary)
        {
            Stream formDataStream = new System.IO.MemoryStream();
            bool needsCLRF = false;
            foreach (var param in postParameters)
            {
                if (needsCLRF)
                    formDataStream.Write(encoding.GetBytes("\r\n"), 0, encoding.GetByteCount("\r\n"));

                needsCLRF = true;
                if (param.Value is FileParameter)
                {
                    FileParameter fileToUpload = (FileParameter)param.Value;
                    //只添加该参数的第一部分，因为我们将直接将文件数据写入流
                    string header = string.Format("--{0}\r\nContent-Disposition: form-data; name=\"{1}\"; filename=\"{2}\";\r\nContent-Type: {3}\r\n\r\n",boundary,param.Key,fileToUpload.FileName ?? param.Key,fileToUpload.ContentType ?? "application/octet-stream");
                    formDataStream.Write(encoding.GetBytes(header), 0, encoding.GetByteCount(header));

                    //将文件数据直接写入流，而不是将其序列化为字符串。
                    formDataStream.Write(fileToUpload.File, 0, fileToUpload.File.Length);
                }
                else
                {
                    string postData = string.Format("--{0}\r\nContent-Disposition: form-data; name=\"{1}\"\r\n\r\n{2}", boundary,param.Key,param.Value);
                    formDataStream.Write(encoding.GetBytes(postData), 0, encoding.GetByteCount(postData));
                }
            }

            //添加请求的结尾。从换行开始
            string footer = "\r\n--" + boundary + "--\r\n";
            formDataStream.Write(encoding.GetBytes(footer), 0, encoding.GetByteCount(footer));

            //将流转储到一个byte[]
            formDataStream.Position = 0;
            byte[] formData = new byte[formDataStream.Length];
            formDataStream.Read(formData, 0, formData.Length);
            formDataStream.Close();

            return formData;
        }

        public class FileParameter
        {
            public byte[] File { get; set; }
            public string FileName { get; set; }
            public string ContentType { get; set; }
            public FileParameter(byte[] file) : this(file, null) { }
            public FileParameter(byte[] file, string filename) : this(file, filename, null) { }
            public FileParameter(byte[] file, string filename, string contenttype)
            {
                File = file;
                FileName = filename;
                ContentType = contenttype;
            }
        }
    }

FormUpload.FileParameter file = new FormUpload.FileParameter(System.IO.File.ReadAllBytes(Path), "123.pdf"); 
       
Dictionary<string, object> dic = new Dictionary<string, object>();
dic.Clear();
dic.Add("file", file);
//提交请求，获得返回结果
var httpWebResp = FormUpload.MultipartFormDataPost(url, "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.3 Safari/535.19", dic);
Stream instream = httpWebResp.GetResponseStream();
StreamReader sr = new StreamReader(instream, Encoding.UTF8);
//返回结果网页(html)代码
string response = sr.ReadToEnd();
sr.Close();