Is there any way to use this in offline?
Press the "Download free" button and make sure that you have the webfonts too. There is a web-fonts-with-css
folder in the downloaded zip. Copy the fonts in your project and modify the paths for the fonts in your CSS to point to the location of the webfonts.
If you open the CSS linked in your questions you'll see that they have some imports with
url('../fonts/fontawesome-webfont.woff2?v=4.7.0')
modify these with the location of the fonts downloaded.
The Fontawesome website itself documents how to download and use the icon kit offline here: https://fontawesome.com/how-to-use/on-the-web/setup/hosting-font-awesome-yourself