def book_post_save(sender, instance=False, **kwargs):

     pdf = Post.objects.get(pk=instance.pk)
     command = "convert -quality 95 -thumbnail 100 %s%s[0] %s%s" % (settings.MEDIA_ROOT, pdf.file, settings.MEDIA_ROOT, pdf.file)
     proc = subprocess.Popen(command,
     shell=True,
     stdin=subprocess.PIPE,
     stdout=subprocess.PIPE,
     stderr=subprocess.PIPE,
     )
     stdout_value = proc.communicate()[0]
post_save.connect(book_post_save, sender=Post)