title: pdf2image - convert_from_path returns an empty image for pdfs with colour
body: <p>I have a collection of pdfs, each containing a scan of an A4 paper, that are different in size. I would like to convert them to an image and fix the resolution of the outgoing image. </p>
  
  <p>My code to convert to jpg (without resizing):</p>
  
  <pre><code>from pdf2image import convert_from_path
  
  filename_in = 'myfile.pdf'
  filename_out = 'myfile.jpg'
  
  jpeg = convert_from_path( filename_in )
  jpeg[0].save( filename_out , 'JPEG' )
  </code></pre>
  
  <p>If the pdf I am trying to convert has any colour in it, the above does not work and the outgoing image is completely white (with non-zero dimensions). Is this a known problem and does a solution exist?</p>
  
  <p>I am using Python 3.7.3.</p>
  
  <p>I am unable to share the pdf files as they contain private information. </p>