Hi Felicity! This is Raech. I'm a fellow Outlook.com user and I'm here to help.
It seems that this is a recent bug as other users have experienced this as well. Fortunately it can be resolved with a few steps.
- Open the email that has the attachment.
- Tap the three vertical dots next to the PDF attachment.
- Select 'Device PDF viewer'.
- Select 'Remember my choice' and then tap the PDF viewer app of your choice on the list.
Kindly check if the problem has been resolved and let me know as soon as possible.
Sincerely,
Raech C.
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