By default if you’re using custom structure /%postname%/ in your WordPress permalink settings, your site will display pretty URLs such as the following example.
The site that has a page with title “About Me” will have https://www.webcazine.com/about-me/ as the URL.
Settings > Permalinks > 
If for some reason you would like to add .htm or .php extensions to the WordPress pages, then you can make use of either one of these 2 plugins.
.php on PAGES – This plugin adds the .php file suffix to all pages of WordPress website.
.htm on PAGES – This plugin adds the .htm file suffix to all pages of WordPress website.
Before you start using these plugins, kindly note that it only works with pages (and not posts), and it makes very little sense to have both plugins activated on your blog.
What other plugins do you use to add .php or .htm extensions to WordPress pages? Let me know in the comments.