How do I uninstall pcloud client properly?

Not official, but you can find most of the files with:

locate pcloud

(If no files are listed, run updatedb.)

Delete the following:

~/.pcloud/
~/.config/pcloud/
~/.config/autostart/pcloud.desktop
~/.local/share/applications/appimagekit-pcloud.desktop
~/.local/share/icons/hicolor/*/apps/appimagekit-pcloud.png

This is unofficial, so some settings might still remain.

I had the same problem and so I decided to contact pCloud directly. I was promptly informed to delete the following files:

1. /home/current-user/.config/pcloud
2. /home/current-user/.pcloud
3. /home/current-user/.config/autostart/pcloud.desktop
4. all instances of "pcloud" you have downloaded /home/downloads/pcloud