This tutorial will explain how to change the default applications in Ubuntu 12.04 Precise for a given type of file.

For general, head to System Settings -> Details -> Default Applications, there you can change default app to launch web, mail, music, video, photo, etc.

For a given file type such as .pdf, .ogg, csv files, you can edit the configuration file /etc/gnome/defaults.list with root privilege:

gksudo gedit /etc/gnome/defaults.list

[Default Applications]
application/csv=libreoffice-calc.desktop
application/excel=libreoffice-calc.desktop
application/msexcel=libreoffice-calc.desktop
application/msword=libreoffice-writer.desktop
application/ogg=rhythmbox.desktop



application/pdf=acroread.desktop
application/vnd.adobe.xfdf=acroread.desktop
application/vnd.fdf=acroread.desktop
application/vnd.adobe.xdp+xml=acroread.desktop
application/vnd.adobe.pdx=acroread.desktop
application/fdf=acroread.desktop
application/xdp=acroread.desktop
application/xfdf=acroread.desktop
application/pdx=acroread.desktop
application/vnd.adobe.air-application-installer-package+zip=AdobeAIR-application-vnd.adobe.air-application-installer-package+zip.desktop