Should I inform the people that I acknowledged in PhD thesis? You write a PhD thesis, in the acknowledgments you thank a series of people from collaborators, to friends, to peers, to professors. Many of these people will never read the thesis so what is the point to acknowledge them? It seems more an exercise for myself. This is different than the acknowledgments in a paper because PhD acknowledgments tend to be more personal and might include friends or people with a non-working relationship that helped in a way that it is not strictly related to the thesis.

Should I send them a copy of the thesis AND mention that I have acknowledged them, or simply let them know in some informal way that I acknowledged them, maybe next time I meet them or sending them a quick email?

It seems a pointless message if they don't know they have been thanked.
Ask yourself, which message do you want to send them?

You do not want to simply inform them about the acknowledgment, you want to THANK them! It's not about your thesis, it's about their help!

How you approach them depends on several factors; do whatever you feel is right.
If it is possible to meet people in person, I prefer this, especially if they have contributed much. If they are further away, perhaps do at least a phone call. You should offer to them send your thesis via email or maybe even give them a printed copy.

In my personal experience, people are always very happy about a "thank you" and it also gives you the chance to stay in contact afterwards.