It is a very strange permission error as it seems to be properly configured. At least for the
How do your trigger the PDF generation? Is it with a script, a cron job, manually from the web page?
I would also try to give also write permission to others, just to discard is not really a permission issue.
sudo chmod ugo+w /opt/bitnami/apps/akeneo/htdocs/var/cache/prod/snappy