I finally figured out that pyPDF can help. I am posting it in case it can help somebody else. 
(1) a  function to locate the string 
def fnPDF_FindText(xFile, xString):
    # xfile : the PDF file in which to look
    # xString : the string to look for
    import pyPdf, re
    PageFound = -1
    pdfDoc = pyPdf.PdfFileReader(file(xFile, "rb"))
    for i in range(0, pdfDoc.getNumPages()):
        content = ""
        content += pdfDoc.getPage(i).extractText() + "\n"
        content1 = content.encode('ascii', 'ignore').lower()
        ResSearch = re.search(xString, content1)
        if ResSearch is not None:
           PageFound = i
           break
     return PageFound
(2) a function to extract the pages of interest 
  def fnPDF_ExtractPages(xFileNameOriginal, xFileNameOutput, xPageStart, xPageEnd):
      from pyPdf import PdfFileReader, PdfFileWriter
      output = PdfFileWriter()
      pdfOne = PdfFileReader(file(xFileNameOriginal, "rb"))
      for i in range(xPageStart, xPageEnd):
          output.addPage(pdfOne.getPage(i))
          outputStream = file(xFileNameOutput, "wb")
          output.write(outputStream)
          outputStream.close()
I hope this will be helpful to somebody else