Health, yes, damage, no. You can set unit's max health trough a simple trigger but when it comes to damage... You could make a permanent cripple buff without slow effect but that would have a % effect. If it's for the hero, you could reduce hero's primary attribute which would directly affect it's damage. You could also try complicating it all with jass.
Permanent Health Bonus +25, I think you can make it negative.
Theres a similar for Permanent Attack Bonus, make it negative.
It's under actions, unit, set unit state, select max life (approximately, i am not able to check it in we right now, but the trigger is there).
It isnt? Damn... But it exists, i assure you, i used it before. Search a bit.