To help increase popularity for Duckduckgo while increasing its reveniue, Duckduckgo should promote its search engine as a search engine that gives some of the money to the poor people just by simply searching on Duckduckgo. How Duckduckgo earns money is through ads.
This is similar to the search engine known as Ecosia
, which uses its ads funds to plant more trees.
Using this idea will increase Duckduckgo's popularity which will for sure increase its revenue.
With increased revenue, Duckduckgo can use the additional money to help improve Duckduckgo's services while introducing new services, which will also increase Duckduckgo's popularity.