Why won't Windows 10 in S mode run an app from Microsoft store?

[Windows 10]

I thought S mode allowed running any app from the Microsoft store. I downloaded Adobe Acrobat Reader DC from the store, and then got a message that I had to leave S mode if I wanted to use it. Why is it in the store if it's not considered safe? How is it possible to tell beforehand whether an app will run in S mode?

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