How do I permanently delete uninstalled files from my iPad?

Apps, when uninstalled, will automatically remove the App and all associated data.

Individual files that you delete using the Files App will initially be moved to the Recently Deleted folder. Here they will remain until automatically deleted in about 30-days - or until they are permanently deleted you. You’ll find the Recently Deleted folder in Files App sidebar.

Similarly, for Photos deleted from your Photo Library, these will be moved to the Recently Deleted folder in the Photos App. The Recently Deleted folder can be found in exactly the same manner - in the Photos App sidebar. Photos will remain here for 30-days - or until permanently deleted by you.

Not sure I understand the question: an app needs to be"installed"; but files generally need an app to make them work. So what kind of files are you referring to?