I typically would not expand a single acronym, and would not use too many. I would give hints, but perhaps I wouldn't do even that.
Consider the TV series "NCIS", you can go half a season without knowing what NCIS stands for. Something to do with the Navy, and Investigating. They have badges. The people they investigate groan and say "NCIS", or seem happy to see them, or ask "What's the Navy's interest here?". But the authors do not go out of their way to inform the audience what the acronym stands for. (Technically when you pronounce each letter, it is called "initialism". Acronyms are when the initials are pronounced as words; like "laser" or "NASA".)
Nor does it make any difference! You may be very proud of your acronym, but in the real world, nobody cares. We have the FBI, NSA, NSF, CIA. We have PHDs, MDs, RNs, some people do not know what those initials stand for either. Same for IBM, GM, the NYSE, the EU, LED. We have all kinds of "xxPD", like NYPD, LAPD, we get it without it ever being spelled out.
These initials just become the name, a label for an organization or a title, and IRL we stop thinking so much about what they stand for and just treat them like a name. So it can feel unnatural for characters to be reciting to themselves, or each other, the meanings of acronyms. Their reactions and thoughts can give oblique hints; If we hear the FBI wants to talk to us, our thought might be What do they want? I don't know anything about any crimes. Maybe about somebody I knew in school?
Remember, show, don't tell. Have your law-abiding characters treat the NDPI like cops (they sound like the FBI), with caution and deference and a little bit of fear. Have your openly law-breaking characters treat the NDPI with suspicion, resistance, and a dollop of anger, hatred and resentment. Have your secretly law-breaking character feel the latter and fake the former.
Have the NDPI act like cops, ask questions like cops, say stuff like cops. A good place to expand acronyms is in formal introductions.
As they entered the meeting room, a man in a crisp business suit sat at the conference table, reading a folder. He looked up, closing the folder and laying it on the table, standing up to greet them.
"Please, take a seat," he said, gesturing to the empty seats near him. "I am agent Malloy. Before we begin, In this particular circumstance I am required to inform you that I am an agent of the National Directorate of Police Intelligence, and lying to an NDPI agent about any matter related to a case is a national crime that may carry a prison sentence of up to five years."
Agent Malloy grinned. "That is never a pleasant way to start a conversation, but it has to be on the tape. I can't lie to the NDPI either."
So we are being recorded, Mike thought. And he can't lie to the NDPI, but he can lie to us. He worried about Kate, she was just reckless enough to get herself into serious trouble here, and he didn't even know what the NDPI wanted.
Something like that; from your POV character.