You should be able to open the file directly -
C-x C-f, and Emacs will display it as you expect.
If you're running GNU/Linux, the pdf-tools package provides a much-improved experience for viewing pdfs than the built-in Emacs viewer. pdf-tools isn't available for Windows, and I think may require some extra work on to get it running on Macs (?).
To do this in an elisp program, use the function
(find-file <path>), where is the path to your file, as a string. e.g.,
(find-file "~/.emacs.d/init.el") will open your init file.