Question

After restoring quarantined file is it still harmful?

Answer 1

yes, in many cases

Quarantine is nothing but a place to store the infected/suspicious files. When you quarantine a file it is deleted from the actual place and moved to the quarantine location (to the path that your anti-virus program has for them).

This is something like keeping a zombie inside a jail. Obviously it is not a threat as long as you don't open the cage.