Newer versions of ForeFlight Mobile require an updated iOS (See this article for details).
Older iPhones or iPads that can be updated past iOS 9.3.5 but cannot be updated past iOS 10.3 are limited to ForeFlight Mobile version 10.6. There was no Pilot's Guide update for v10.6 so for reference here is a link to the Pilot's Guide for ForeFlight Mobile version 10.5.
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