Fixed django#18440 -- Pointed out that ReportLab is not thread safe. Thanks, jens@lundstroem.com

Author: adrianholovaty

docs/howto/outputting-pdf.txt

@@ -100,6 +100,12 @@ mention:
 * Finally, it's important to call ``showPage()`` and ``save()`` on the PDF
  file.
 
+.. note::
+
+ ReportLab is not thread-safe. Some of our users have reported odd issues
+ with building PDF-generating Django views that are accessed by many people
+ at the same time.
+
 Complex PDFs
 ============