When opening password protected PDF's via the Outlook app and Outlook on my Mac, all landscape slides are show in portrait so that the contents are cut in half.
I noticed this happened a few weeks ago when I installed the latest iOS 13.1.3 and macOS Catalina 10.15 updates.
If I remove the password and send the PDF to myself via Outlook again, the slides are viewed in the correct format.
I have a workaround and that is to save the PDF to One Drive first but it's a bit of a nuisance.
If anyone has a fix for this, please let me know.