How can I browse some document-like apps offline?

Some Android apps are actually some kind of documents/ebooks, but it seems that I can't browse it offline. for example, it seems that when my phone is not connected to a network or mobile data service, the following app doesn't load - [LeetCode Cracker](https://play.google.com/store/apps/details?id=me.zhujiaqi.leetcode) How can I browse such apps offline? Thanks.