A Microsoft file hosting and synchronization service.
I am using the OneDrive web version (I do not use desktop version) and am also experiencing the same problem using my MacBook Pro after updating to macOS Sequoia 18.6. PDF and JPG files open for a second and then disappear. WORD and EXCEL files open as expected. This behavior occured using both Safari and Google Chrome browsers. Any PDF and JPG files in OneDrive that are not within PERSONAL VAULT open without problem. This behavior does NOT occur within PERSONAL VAULT using the OneDrive app on my iPhone 13 Pro Max (which I also updated 8/6/25 to iOS 18.6).
UPDATE: I tried the recommendation about navigating to Safari settings>Websites and clearing all automatically stored websites that I allow downloads from. Plus, in System Settings>Network, toggle Firewall off then back on. Then restart. Safari still is not working, but Chrome has started working.