Keep the successful result in view
A title and cover in the Jellyfin library show that the server has catalogued an item. They do not prove that every client includes a reader for its format. Record the file format and the exact client where the reading task fails before changing metadata.
Use a small client acceptance check
Choose one authorised sample of the same format. Confirm that it is the expected edition and that the original file is intact. Check the client's own documented capabilities and any visible error. If another intended client is available, compare the same sample without treating that comparison as proof about all devices.
Avoid downloading an unofficial reader build or changing the entire library because one interface cannot open the sample. Likewise, do not promise that a conversion will preserve layout, annotations or navigation without reviewing those requirements and the applicable file rights.
Document the usable route
Keep a short note containing format, client version, server version and the task actually checked: opening, navigating and returning to position where supported. If the requirement is unmet, state that limit. This is more useful than labelling the whole library compatible because the server displays its cover.
Questions answered
Is an unreadable item necessarily misidentified?
No. The catalogue can be correct while the intended client lacks the required reading capability.
Can I infer all book formats work from one successful PDF?
No. Review the specific format and reading task.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.