Everyone is confused between the two files because bootstrap.css and bootstrap.min.css.
The Both two files comes up with the Bootstrap framework. Having same css code. they why we have two css files.
The main differnce is that the bootstrap.min.css file is minified.
MINIFIED means all the whitespace and the Xtra characters have been removed. The file starts with one line and ends with one line.
The purpose of using this file to reduce the size of the file and to speed up the Website. So by using bootstrap.min.css website speed got faster rather using bootstrap.css.
But there is hard to change in the bootstrap.min.css file because code stars and ends in one line.