It's not the default on my Ubuntu, and I don't even see it in apt or snap. I wonder if perhaps you've confused it with another program, or are running an Ubuntu derivative.
When I go to the "Files" app and open a PDF, it opens the PDF in my default web browser at photopea.com, i.e. it's not an "app on my machine" so it doesn't show up in the apt list. Photopea does show up us as one of the options when I click on "Show Applications."
When I go to the "Files" app and open a PDF, it opens the PDF in my default web browser at photopea.com, i.e. it's not an "app on my machine" so it doesn't show up in the apt list. Photopea does show up us as one of the options when I click on "Show Applications."