Should I inform the people that I acknowledged in PhD thesis?

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.

I’ve been acknowledged in a few dissertations, and didn’t see until I read their dissertations while looking at format for my own! I wasn’t mad they didn’t tell me, but I wish I could've been able to express my happiness to collaborate with them in person and not a text, since they already have left the institution. I vote you let them know! Maybe email them the page where they’re mentioned.

I handed over a copy of my printed thesis to every friend I mentioned in the acknowledgments. In addition, I wrote a kind of thanking latter or dedication to the recipient thanking for their support or whatever. When handing over the thesis copy, I verbally thanked them and told they were mentioned in the acknowledgments.