A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Chinua Mosley
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and to my knowledge, the behavior observing when opening the Anyone with this links in the PDF's is strange because the same links are able to open without any sign-in prompt when opening in the other OS devices.
Since it happens only with iPhone devices, I believe the problem needs to be further checked by the related development team by collecting the logs. I suggest you raise a support ticket from the Office 365 admin center, where the related team will investigate the problem further by collecting the logs.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket.
Apologies for redirecting you to the related development team support as the moderators in this community have limited resources to collect and investigate the logs, and to get the fast and better assistance we requested for it.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia