This is not a topic specific to the KDE edition, so moving it here.
Assuming you selected to have an encrypted home folder during installation, you have not enabled automatic login, and have a long enough password your personal documents are relatively safe. You can gauge your password strength on various websites, such as http://www.passwordmeter.com/
(though I advise to never type your actual password, just something that is similar to it). Longer passwords are better than shorter passwords with w31rd characters.
While any tech savvy person that has physical access to your installation should be able to login as root very easily, it is not so easy to gain access to your encrypted files (they will have to guess your password). Changing your password on your user account (so they don't have to guess it), which root can do, will effectively destroy the files in your user's encrypted home directory.
With an encrypted home folder, your data should be mostly safe.
Note that if somebody has physical access to your installation, as root they can install some backdoor program so that when later you again log in they are able to get your personal documents that way. So if you are paranoid or carrying highly sensitive documents, you might want to increase your protection to prevent others from installing a backdoor on your installation. You may want to install with full disk encryption, instead of just home folder encrypted, though the Linux Mint installer currently can't do that automatically. There are various topics on that to be found however, such as viewtopic.php?f=46&t=103843
. You may also want to password protect your GRUB2 boot loader, for example as shared here http://www.noobslab.com/2012/02/passwor ... -with.html