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I understand you have changed the old email account in Outlook to Office 365. When you send email from Adobe PDF Reader, the application will open Outlook by default. Since Office 365 is a cloud product, there is no software installation for it in your computer, so you cannot select Office 365 to open file in the control panel directly.
Currently my recommendation is to create a new Outlook profile and connect it to your Office 365 account. After that, you will be able to use Office 365 email services in Outlook as before. Please note that only Outlook 2007 SP2 or above versions are supported in Office 365. You can connect it with the steps in Connect Outlook to This Account.
By the way, PDF is supported attachment type in Office 365, please see Working with Attachments.
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