How to write string value to pdf file in unity3d

I am getting txt file without any additional namespace but i am not getting in pdf formate. please any one ......

Thanks.

string message="hi this is pdf formate"; System.Text.UnicodeEncoding encod = new System.Text.UnicodeEncoding();

                 byte[] byteData = encod.GetBytes(message);
                 
                 System.IO.FileStream oFileStream = null;
                 oFileStream = new System.IO.FileStream("D:\\DoneFile.pdf", System.IO.FileMode.Create);
                 oFileStream.Write(byteData, 0, byteData.Length);
                 oFileStream.Close ();