Encrypting a single file on iOS

Your iPad has an encrypted filesystem. All files locally stored on your iPad are encrypted at rest by default. As such, unless anyone that has access to your iPad has knowledge of your iPad Passcode, no data can be accessed.

Outside of centrally managed Educational or Enterprise/Business organisations, iOS/iPadOS are single-user devices. Given that only you should have knowledge of the Passcode, nobody else can access your files.

Should someone have physical access to your device - and attempt to guess your Passcode - they have only a few attempts to guess the correct passcode before the iPad will automatically completely lock or erase all locally data. Before this occurs, waiting times are invoked that slow down the opportunity to enter another Passcode.

Unless you are sharing your iPad with another User, your locally stored data and files are fully protected from discovery. If you are intentionally sharing your iOS/iPadOS device with anyone, you perhaps have greater risks to your data than prevent to access to a single file.

Some Apps do have the ability to password-protect individual files, this facility being applied by the App and not the Operating System. There are third-party App that can be downloaded from the App Store that offer additional encryption capabilities - however, their effectiveness and efficacy may be unproven or unreliable.

In conclusion, unless you fully understand and can explain the security issue that you are attempting to mitigate - and or the security target - it is very difficult to offer specific advice.