Be default PDF files are opened by Docviewer in Ubuntu distributes. Yet, there is a more convenient PDF tool, aptly named pdf-tools that works well with Emacs. The installation of pdf-tools is straight forward. Below is what I did.
sudo apt install libpng-dev zlib1g-dev sudo apt install libpoppler-glib-dev sudo apt install libpoppler-private-dev
After that, go to Emacs and use the package-install to install
package-refresh-contents package-install pdf-tools
Finally, add the following line to the .emacs file. And make sure that if you are displaying line numbers in emacs, make sure it is disabled in pdf-view-mode. Here what I used is to just enable line number display in prog-mode.
(pdf-tools-install) (add-hook 'prog-mode-hook 'linum-on)