You want to use the dist target specified in the package.json file as main entry.
In this case, you are looking for the umd build (dist/umd/popper.js)
What’s UMD?
The UMD pattern typically attempts to offer compatibility with the most popular script loaders of the day (e.g RequireJS amongst others). In many cases it uses AMD as a base, with special-casing added to handle CommonJS compatibility.
This means that an UMD bundle can be loaded via <script> tag and get injected inside the global scope (window), but also work if required with a CommonJS loader such as RequireJS.