FIXED logic of verbose flag in PDF post processor

IPython/nbconvert/post_processors/pdf.py

@@ -45,7 +45,7 b' class PDFPostProcessor(PostProcessorBase):'
             command = [c.format(filename=input) for c in self.command]
             self.log.info("Building PDF: `%s`", ' '.join(command))
             with open(os.devnull, 'wb') as null:
-                stdout = null if self.verbose else None
+                stdout = null if not self.verbose else None
                 for index in range(self.iteration_count):
                     p = subprocess.Popen(command, stdout=stdout)
                     p.wait()