Emacs support library for PDF files.
PDF Tools is, among other things, a replacement of DocView for PDF files. The key difference is, that pages are not pre-rendered by e.g. ghostscript and stored in the file-system, but rather created on-demand and stored in memory.
This rendering is performed by a special library named, for whatever reason, poppler, running inside a server program. This program is called epdfinfo and its job is it to successively read requests from Emacs and produce the proper results, i.e. the PNG image of a PDF page.
Actually, displaying PDF files is just one part of PDF Tools. Since poppler can provide us with all kinds of information about a document and is also able to modify it, there is a lot more we can do with it.