How to properly handle encryption for files?

Scenario: There is a mobile application that uses a database.txt file, which is inclusive to the download of the application and therefore offline accessible.

Now I need to make sure that the user cannot access, copy or do something else with that text file. Only the app should be able to access it.

From what I learned so far I could encrypt the file on Start and decrypt it whenever the app needs to use it. I cannot find clear information though, if the file is secure while the app is on the device but never used. Also it seems quite expensive performance wise to always have to decrypt the whole file when using it.

What is the proper way to secure that txt file?