打开：~/.config/mimeapps.list
去掉以下几项：
image/pdf=display-im6.desktop
image/pdf=display-im6.q16.desktop;display-im6.desktop;
通过设置以上即可解决问题，再次在PDF文件属性上设置其它为默认打开程序即可。
其它配置文件说明，与上面无关：
全局配置 /etc/gnome/defaults.list /usr/share/applications/mimeinfo.cache 个人配置 ~/.local/share/applications/mimeapps.list ~/.local/share/applications/mimeinfo.cache
参考：
https://askubuntu.com/questions/570830/unable-to-restore-envince-as-default-pdf-viewer
https://ubuntuforums.org/showthread.php?t=2329806
https://answers.launchpad.net/ubuntu/+source/imagemagick/+question/263052
https://askubuntu.com/questions/591425/why-do-pdf-documents-open-with-imagemagick
https://unix.stackexchange.com/questions/226857/set-default-pdf-reader