Hi Luen, this is Kimberly. I'm glad to assist you today.
Your Outlook PDF preview works in Incognito because your normal browser profile has corrupted Outlook site data or settings.
Fix steps (in order):
Check PDF settings in Chrome/Edge and keep inline viewing enabled.
Clear Outlook site cookies/data (outlook.office.com, office.com, live.com).
Turn off hardware acceleration in browser settings.
Reset all experimental flags.
If still broken, create a new browser profile to replace the corrupted one.
Should you have more questions, please let me know.
Best regards,
Kimberly