The train enters a tunnel just as you open your notes. The filename is there, but the document asks to download. The folder was showing you what existed in the cloud, not necessarily what was stored locally.
Mark the actual working set
Use your service’s option to keep the required files on the device. OneDrive Files On-Demand, for example, distinguishes online-only items from files kept locally. Check the status after the download finishes; selecting the option is only the start.
Include supporting files: attachments, linked images or reference documents that your main file depends on. Downloading an index or shortcut does not necessarily bring those with it.
Do a small offline rehearsal
With enough time to fix problems, disconnect from the network and open the files in the application you intend to use. Try the task itself, not just a preview. An application may need its own sign-in or online features even when the document is present.
Check battery, available storage and any changes made since the initial download before departure.
Plan what happens on reconnection
If you edit while offline, avoid having several people change the same material without a plan. On reconnecting, let synchronisation finish and inspect any conflict notices before clearing local copies. Offline availability helps you work without a network; it is not a separate backup or a guarantee that every collaborative feature will work away from the service.