I downloaded and ran a binary from their website (not a debian package) with these instructions. I can't seem to find an uninstall.
Not official, but you can find most of the files with:
(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.