wordpress present itself instead of a 404 Error page

I have wordpress install in a subfolder on my domain name such as https://www.google.com/word. The main site is shown at the root of the domain name https://www.google.com. I put some of the wordpress posts on non-wordpress php pages in the root of domain name. I noticed that if I misspell the name of php page, my blog show up. I should get a Error page if the php do not exist, instead of my blog showing up.
How do I make make a 404 error page should show up in the root domain name, where the blog isn’t locate?