Can open-source documentations be monetized?

I came across an Android app called Node.js Docs by an organisation named NextLabs.cc. Here is the app in Google Play store.

And to my surprise, they have monetized the contents!

Specifically,

• There are banner ads in the app.
• To access the documentation of each version for one day, you need to watch a 15 second video.
• You can buy the whole app for a very little amount but it's monetization nonetheless.

I don't know much about open-source licenses and monetization so I wanted to ask it here.

Is this legal?

I'd like to know more about licenses and how all these work.

Comments

[manish srivastava]

There are different types of licences. The best place to know about a license is GitHub. Try to select one in empty repo.

A Software is open source when 'source' is open. If you can build another app by use of source code.

Then there is another thing FOSS: it's free open source software. The word free has lots of meaning here.

What they are doing they are providing documents of open source project: nodejs but the app itself is not. They are just showing ads... Wherever possible

[Loralighte]

To my understanding NodeJS is open-source, and these people profit on the docs and not the actual project, so from what I see here this is absolutely legal. You can profit on an open-source project, even if you don't make it, as long as:
A: You follow the license
B: You follow trademark and copyright law

This case seems like both are true.

[Abhijit Hota]

I see. Thanks for the answer!