It is essential that you sign out of the iPad once you have finished.
This is for security, but also to sync your account to the cloud and allow access when you sign in next time (and also to access your files online from other devices such and the PCs, via iCloud.com)
Swipe DOWN from the top-left of the iPad screen.
Select Sign Out (bottom-right)
You will then be signed out and your account and files synchronised to the cloud.