tumblerto generate thumbnails, and this is controlled by a config file. So, begin by making a local copy of the config file:
$ mkdir ~/.config/tumbler/ $ cp /etc/xdg/tumbler/tumbler.rc ~/.config/tumbler/
and then edit the local
tumbler.rcfile as needed to disable the unwanted thumbnailers:
# PDF/PS thumbnailer [PopplerThumbnailer] Disabled=true Priority=1 Locations= MaxFileSize=0
You'll also have to delete the unwanted thumbnails from
~/.thumbnails/normal. I have a
~/.thumbnails/largeas well, and those might need to be deleted too.