In Notepad++, use replace all with regular expression. This has advantage over conversion command in menu that you can operate on entire folder w/o having to open each file or drag n drop (on several hundred files it will noticeably become slower) plus you can also set filename wildcard filter.
(\r?\n)|(\r\n?)
to
\n
This will match every possible line ending pattern (single \r, \n or \r\n) back to \n. (Or \r\n if you are converting to windows-style)
To operate on multiple files, either: