So, any time you double-click on, or otherwise launch, one of those files, it'll open in Preview.
Preview is the default app for viewing images and PDF files on your Mac. How to view photos and PDFs in Preview on Mac While it's primary purpose offers you a quick and easy way to see various documents, it has several pretty cool editing and markup features you may not even be know are there. Preview is the default application in macOS for image files, like JPG and PNG, and PDF files.
