Uploading php files with WordPress installed

Okay so I have WordPress installed (Plesk) in the main domain url (httpdocs), but I would also like to add pages of my own unrelated to WordPress in the same domain name. I am unsure how to do this without them interfering with each other since if I add a new index.php to the folder, WordPress stops working entirely and I can’t access the domain page.

Am I stuck with only being able to use WordPress in the domain since it’s installed in the main url?

EDIT: I read that creating a sub domain is what I’d need to make this work?