I recently switched back to using Ubuntu as my desktop OS, and was intrigued to see that Photopea was the default app to open many file types, including PDF.
I thought that was odd because while Photopea is free-of-charge, it's not completely open source [1].
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."
If you go to www.Photopea.com and press More - "Install Photopea", it will install the so-called "PWA". This allows starting Photopea with a homescreen icon, in a window without a browser UI. It also allows associating a website with certain extensions, so your system will "open files in a website". It should work on every device/OS/browser (in the long run). You can uninstall it in your browser under about://apps
I thought that was odd because while Photopea is free-of-charge, it's not completely open source [1].
[1] https://github.com/photopea/photopea