Acrobat document cloud greyed out when trying to ‘save to file’ in iPad mail

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Acrobat document cloud greyed out when trying to ‘save to file’ in iPad mail.

I want to save an PDF attachment in Apple iOS mail to my Acrobat Document Cloud, but th oprion is greyed out (see screenshot). I'm signed in to Acrobat DC and it's all active, but still can't make it a live option!!
When I open acrobat on my iPad I can see my ADC folder, so it's there and active - it just won't let me

sabe to it from my Apple mail, so frustrating.

Any ideas?