Hello JoeyWin
Welcome to Microsoft Community.
I understand that you are having trouble on Outlook's pdf preview cannot be adjusted.
In fact, Outlook itself does not support the preview of PDF files. Check whether your PDF preview tool is working properly through the website below.
Outlook PDF preview error: This file cannot be previewed because there is no previewer installed for it - Microsoft Support
I am using the pdf preview of Edge browser, and in my Outlook 2021 Professional Plus, I can find the button in the picture below, which helps us scale the pdf to a suitable size.
(If your interface is different from mine, then this may be caused by a different previewer, try switching your pdf previewer to Edge.)


I hope my steps are helpful to you.
Feel free to reply if there is any progress on the issue.
Sincerely,
Hardyjoe.J|Microsoft Community Support Specialist