CPI typically stands for several different terms in various contexts, and it's not immediately clear which one you're referring to. Here are a few possibilitie

Consumer Price Index

The Consumer Price Index measures the average change over time in the prices paid by urban consumers for a market basket of consumer goods and services. If you are looking to open a file related to CPI data, it could be in a format like Excel, or a file format used by statistical software. In this case, you would