Read contact from StringBuilder object

Your question is not clear enough. If the problem with extracting text from the PDF using iTextSharp, see the following discussion:
http://stackoverflow.com/questions/4711134/itextsharp-text-extraction

Note that, I didn't try iTextSharp myself before, but I am getting the text from PDF files using PDFdocument features using leadtools.

If the problem is with getting the numeric text (numbers) from the original string, try this code:
/****************************************************/
StringBuilder sb = new StringBuilder();
sb.Append("PH NO 456456121");
int _lastspaceindex = sb.ToString().LastIndexOf(' ');
string numeric_str = sb.ToString().Substring(_lastspaceindex, sb.Length - _lastspaceindex).Trim();
Console.WriteLine(numeric_str);
/****************************************************/