It looks like you are trying to open in a different PDF viewer and not in Acrobat Reader. According to the 4th screenshot, go to the settings of the application and make it default and then try to open the PDF from outlook.
so I go into settings, tap the app, tap not set as default and there is nothing to change. Taping on open supported links is set to In this app. clear defaults greyed out. Nothing under supproted web addresses apart from some adobeacrobat links.
Could you confirm that if you try to open any other PDF which is already saved on your mobile, does that ask which application you want to use to open it with? If yes, then select Acrobat and and then select "Always".