There is the navigator.storage.persist() API that allows the storage for a site/origin to become persistent (and maybe increase the limit) and I think it would very useful to use it on this project to prevent the browser from deleting the docs if you are running low on storage or you haven't used this page in a while, or even to increase the storage limit for people that download many docs for offline use.
Of course it should be optional. I think the best way to add this would be adding a button in the offline data section that the user can choose to click if they want to make the storage persistent.

https://developer.mozilla.org/en-US/docs/Web/API/StorageManager/persist