Hello S Heim,
Greetings!
Thank you for posting on Microsoft Community!
I understand that you want to open the PDF file in New Outlook. From the screenshot you shared, you've actually downloaded the PDF attachment files on the email. Hence you could only open and view the PDF with the tool installed on your computer, if you want to change the default app to open PDF
- Search for "Default apps" and open it on Windows search bar.

- Scroll down and select "Choose default apps by file type"
3. Find the file type of .pdf and select the default app.

If you want to preview the PDF file in New Outlook, you may just simply click on that attachment file and select "Preview".

f the above mentioned information could not address your concern, feel free to reply and provide more information to us. We will continue to support you.
Thank you for your time and cooperation. Wish you have a nice day!