Emacs includes functionality to act as an image and document viewer via its Docview feature.
Most usually the way to use this feature is to simply find a file of an appropriate type, e.g. a PNG image or a PDF document.
When in Docview-mode(or M-x image-toggle-display) will toggle between viewing the image or document graphically and viewing it in text format. This can be useful for directly modifying PDF files, for instance.
Docview mode typically works out of the box on GNU/Linux systems. If you are running Emacs on Windows it likely requires some additional libraries to be manually installed.