Para desativá-lo em todo o sistema, comente a linha Exec= em /usr/share/thumbnailers/evince.thumbnailer :
sudo sed -i 's/^Exec=/# &/' /usr/share/thumbnailers/evince.thumbnailer
user@user-X550CL ~/tmp % sudo sed -i 's/^Exec=/# &/' /usr/share/thumbnailers/evince.thumbnailer
user@user-X550CL ~/tmp % cat /usr/share/thumbnailers/evince.thumbnailer
[Thumbnailer Entry]
TryExec=evince-thumbnailer
# Exec=evince-thumbnailer -s %s %u %o
MimeType=application/pdf;application/x-bzpdf;application/x-gzpdf;application/x-xzpdf;application/x-ext-pdf;application/postscript;application/x-bzpostscript;application/x-gzpostscript;image/x-eps;image/x-bzeps;image/x-gzeps;application/x-ext-ps;application/x-ext-eps;application/x-dvi;application/x-bzdvi;application/x-gzdvi;application/x-ext-dvi;image/vnd.djvu;application/x-ext-djv;application/x-ext-djvu;image/tiff;application/x-cbr;application/x-cbz;application/x-cb7;application/x-cbt;application/x-ext-cbr;application/x-ext-cbz;application/x-ext-cb7;application/x-ext-cbt;application/oxps;application/vnd.ms-xpsdocument
Para reativá-lo, descomente-o novamente:
sudo sed -i 's/^# \(Exec=\)//' /usr/share/thumbnailers/evince.thumbnailer
user@user-X550CL ~/tmp % sudo sed -i 's/^# \(Exec=\)//' /usr/share/thumbnailers/evince.thumbnailer
user@user-X550CL ~/tmp % cat /usr/share/thumbnailers/evince.thumbnailer
[Thumbnailer Entry]
TryExec=evince-thumbnailer
Exec=evince-thumbnailer -s %s %u %o
MimeType=application/pdf;application/x-bzpdf;application/x-gzpdf;application/x-xzpdf;application/x-ext-pdf;application/postscript;application/x-bzpostscript;application/x-gzpostscript;image/x-eps;image/x-bzeps;image/x-gzeps;application/x-ext-ps;application/x-ext-eps;application/x-dvi;application/x-bzdvi;application/x-gzdvi;application/x-ext-dvi;image/vnd.djvu;application/x-ext-djv;application/x-ext-djvu;image/tiff;application/x-cbr;application/x-cbz;application/x-cb7;application/x-cbt;application/x-ext-cbr;application/x-ext-cbz;application/x-ext-cb7;application/x-ext-cbt;application/oxps;application/vnd.ms-xpsdocument