sudo sed -i 's/^Exec=/# &/' /usr/share/thumbnailers/evince.thumbnailer

sudo sed -i 's/^# (Exec=)/1/' /usr/share/thumbnailers/evince.thumbnailer

cd /usr/share/thumbnailers
sudo nano evince.thumbnailer
# enter superuser password
# then uncomment the Exec line by entering a # sign as the first character in the line
# exit nano with Ctrl-X, press y, press Enter, DONE!