If UIPasteboard object is set 'persistent', it seems that items of UIPasteboard are stored in database or something like that. So is there any hint to me to find where the data is stored in system? Jailbreak. Thanks in advance.
Pasteboard is managed by com.apple.UIKit.pasteboardd daemon located at /System/Library/Frameworks/UIKit.framework/Support/pasteboardd. It doesn't run all the time, only when somebody actually needs it i.e. uses copy/paste.
It does use a persistent storage - /var/mobile/Library/Caches/com.apple.UIKit.pboard/pasteboardDB binary property list. Even if you SIGKILL the daemon it will restore the pasteboard contents from the database.
pasteboardDB root object is NSArray. Each element is a NSDictionary object that's bound to specific bundle id and name. For example, general copy/paste uses shared storage - dictionary has bundle->com.apple.UIKit.pboard and name->com.apple.UIKit.pboard.general keys. Under items key there is an array of pasteboard items. Yoy can try to copy some text and you will find it there as unencrypted UTF8 string stored as NSData object.