[Edited response]
Hi @marko_4822
We hope you are doing well. Thanks for reaching out. We are sorry for the trouble you are experiencing.
++ Adding to the discussion
As described by @creative explorer The AI Assistant isn't a standalone app. It's an integrated feature within the Acrobat Reader Mobile app (or the paid Acrobat desktop app).
To use the Acrobat AI assistant on iPhone mobile device, you need to download the Acrobat Reader mobile app (Freeware) from the App Store and install it on your iPhone. Once done, open any PDF file using the Acrobat Reader mobile app, and then select AI Assistant.
Alternatively, you can also access AI Assistant from the contextual menu. Select the text for AI Assistant. The contextual menu opens, then select Ask AI Assistant.
Also, make sure the Generative AI is enabled from the Acrobat Mobile app preferences. Tap on the profile > Preferences > Enable 'Generative AI features in Acrobat'

To know more about using generative AI features in Acrobat Mobile, please check the steps shared in the help page https://helpx.adobe.com/acrobat/using/generative-ai-features-in-acrobat-mobile.html
Let us know if you still experience any trouble and need more help.