string readPath= "D:\\Read.html";
string writePath= "D:\\write.html";
StreamReader sr = new StreamReader(readPath);
string html = sr.ReadToEnd();
StreamWriter sw = new StreamWriter(writePath);
sw.WriteLine(html);
или
string path = "D:\\Read.html";
string path1 = "D:\\write.html";
html=ReadAllText(path);
File.WriteAllText(path1, html);