Edit file mimeapps.list nano ~/.local/share/applications/mimeapps.list change the row application/pdf=kde4-okularApplication_pdf.desktop kde4-okularApplication_pdf.desktop (the full name of okular) Edit file defaults.list sudo nano /etc/gnome/defaults.list change the rows as follow application/pdf=kde4-okularApplication_pdf.desktop application/vnd.adobe.xfdf=kde4-okularApplication_pdf.desktop application/vnd.fdf=kde4-okularApplication_pdf.desktop application/vnd.adobe.xdp+xml=kde4-okularApplication_pdf.desktop application/vnd.adobe.pdx=kde4-okularApplication_pdf.desktop application/fdf=kde4-okularApplication_pdf.desktop application/xdp=kde4-okularApplication_pdf.desktop pplication/xfdf=kde4-okularApplication_pdf.desktop application/pdx=kde4-okularApplication_pdf.desktop …