When configuring PDNS it is important to understand the difference between forward-zones and forward-zones-recurse. In general, it is safer to use the forward-zones-recurse if you are unsure of the status of the upstream server you want to configure. You can find more detailed documentation on the settings for forward-zones here and forward-zones-recurse here or use the URL below
If you have an entry in the Forward-zones that is not an authoritative server you will experience "SERVFAIL" errors when using dig for the related domains along with seeing errors in browser when accessing the URL similar to "Error: DNS Lookup Errors".
When using a non-managed upstream server in your Forward-zones, it is best to configure it as a forward-zones-recurse. Since, something could change on the infrastructure of their name servers and may break the lookup on your usage of those sites. When in doubt, use the forward-zones-recurse in the PDNS Configuration.
BEST PRACTICE: DO NOT use forward-zones ALWAYS use forward-zones-recurse.