How to convert from string into pdf?

using (Stream stream = ... fetch the stream from somewhere)
{
    byte[] buffer = new byte[stream.Length];
    stream.Read(buffer, 0, buffer.Length);
    File.WriteAllBytes("foo.pdf", buffer);
}

and if this RESTful service talks HTTP you could use a WebClient:

using (var client = new WebClient())
{
    client.DownloadFile("http://example.com/api", "foo.pdf");
}