Hello! I had a question about file naming convention for file which contains multiple classes or only top-level declarations. https://kotlinlang.org/docs/reference/coding-conventions.html#source-file-names - use name with a upper first letter, but https://github.com/JetBrains/kotlin - use name with a lower first letter. What would be better? I prefer second way, but in little confuse why official kotlin github no follow official kotlin convention

Where did you find that

use name with a lower first letter

?

It depends(?). For example https://github.com/JetBrains/kotlin/tree/master/build-common/src/org/jetbrains/kotlin/build

That's part of the build, not (directly) part of Kotlin itself. Anyway, I'd highly recommend following the convention from the docs.

It seems that if it is a single class file, then it matches its name of the class (which start with capital letters), But if the file contains multiple declarations it uses initial lowercase.

The link above to collections is a folder. Look at all the files within it and they're all uppercase first.