Is it possible to put pin-code restriction on a folder/file, so I can "hide" an app that I don't want anybody to access? Or is it just possible to put a pin-code restriction on an app such as the mailapp

'Settings', 'General', 'Restrictions' allows you to put a passcode lock on Safari, Facebook, iTunes, deleting apps, installing apps - and that's it.

Passcode restricting access to the iPad's other native apps - such as Mail - or the iPad's native folders is not possible. Some third-party apps will protect data stored in their proprietary folder structure, but they cannot passcode protect the iPad's native folders, because these are under the control of the iPad's operating system iOS and all programs (apps) on the iPad are 'sandboxed', which means they have very limited ability to interact with each other and all of this interaction is strictly controlled by iOS. Upside - absolutely no possibility of a virus. Downside - what you want to do is not possible unless Apple decide to introduce it in the next version of iOS.

Sorry to be the bearer of bad tidings....

Tim
Scotland