title: Error using comtypes.client to convert a .pptx file into .pdf
body: <p>I'm trying to convert a .pptx file to a pdf file. So far I got:</p>
  
  <pre><code>PP_FORMAT_PDF = 32
  
  def _convert_powerpoint2pdf(input_path, output_path):
      try:
          import comtypes
          import comtypes.client
      except ImportError as error:
          raise 
      else:
          powerpoint = slides = None
          try:
              comtypes.CoInitialize()
              powerpoint = comtypes.client.CreateObject('Powerpoint.Application')
              slides = powerpoint.Presentations.Open(input_path)
              slides.SaveAs(output_path, FileFormat=PP_FORMAT_PDF)
          except WindowsError as error:
              raise
          except comtypes.COMError as error:
              raise IOError(error)
          finally:
              slides and slides.Close()
              powerpoint and powerpoint.Quit()
              comtypes.CoUninitialize()
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:/Users/Mathias/git/pdfconv/tests\test_converter.py", line 89, in test_convert_presentation2pdf_pptx
      pdfconv.converter.convert_presentation2pdf(input_path, output_path)
    File "c:\users\mathias\git\pdfconv\pdfconv\converter.py", line 116, in convert_presentation2pdf
      _convert_powerpoint2pdf(input_path, output_path)
    File "c:\users\mathias\git\pdfconv\pdfconv\converter.py", line 179, in _convert_powerpoint2pdf
      slides.SaveAs(output_path, FileFormat=PP_FORMAT_PDF)
  _ctypes.COMError: (-2147467259, 'Unbekannter Fehler', (u'Presentation (unknown member) : PowerPoint kann ^0 auf ^1 nicht speichern.', u'Microsoft PowerPoint 2013', u'', 0, None))
  </code></pre>
  
  <p>Almost the same code works perfectly fine for Word and Excel. Has anyone an idea what I am doing wrong?</p>