Hi, @Joda1
I looked into this on a few devices and asked around but have not heard of or seen PDFs not opening for some users, but not others, especially related to annotations. It would make more sense if it was all or nothing.
I think support would need to look at a specific case and do a deep dive. I would suggest you and/or your students work with your local support to troubleshoot. I would also suggest students Report an Issue under the Help section of the app:
How do I get help with the Student app on my iOS device?
I'll keep looking around, but that is my best suggestion at this point.
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