docview is a built-in document viewer for Emacs that can display dvi, ps, and pdf files. It isn’t as good as most PDF readers, but in a lot of cases it’s good enough without having to switch to a different application. It works by converting the files to a series of ‘png’ images, one per page, which Emacs can display. This means that you can’t edit them or copy text from PDF files, but if you only need to view them it works.
docview is enabled by default in emacs - opening a pdf file as a normal file will turn on docview-mode. The only customization I currently have for it is to turn on auto-revert for pdf files; I use this when creating a PDF file using latex and wish it to be auto-updated. There’s not much else I can think of that would improve this mode by the use of hooks - mostly to make it better Emacs would need native PDF support.
(add-hook 'doc-view-mode-hook 'turn-on-auto-revert-mode)
Using docview is fairly simple: n/p will move to the next or previous page and C-n/C-p moves up or down on the current page. You can zomm in and out with + and - respectively. Other normal movement keys are rebound to something appropriate - M-> will go to the last page, for example. You can also search for text with ‘C-s’ and ‘C-r’, but this will only take you to the correct page and message you the line the text is on - it won’t highlight the text for you, so it is rather subpar.