Using Classic Outlook on Windows for personal email, calendar, and contact management
Hi BuckyBear3819,
Thank you for posting your query on Microsoft Community. However, PDF file type attachment can be previewed in Outlook Web App. See View and edit an attachment in Outlook Web App
May I know which browser are you using to sign in to Outlook Web App? Please provide us with the detailed information including version about internet browser.
Make sure that browser are latest updated and also Adobe Acrobat Reader the default program used to open PDF files.
For your reference: Supported browsers for Outlook on the web for business
Does this issue happen to specific or every pdf attachments?
Please try to clear/reset the internet browser cache to check this issue.
Have tried to check this issue via using different browser? If not, please try to use different browser (e.g. IE, Firefox or Chrome browser) or InPrivate/ Incognito browse window to see whether it is a specific browser side issue.
Best regards,
Shyamal