php files

I downloaded the opt in plugin above mentioned version 1.1.8 but I cant seem to open the php files…I get a message that software is not available to open the files and no luck when I search the web…I am a bit techie challenged. Just wanting to have opt in forms on my wp sites I use aweber but have not set up opt ins before

https://ww.wp.xz.cn/plugins/optin-forms/