pearOS layers its own keyring (pearos-keyring) on top of Arch's and CachyOS's. If pacman starts complaining about invalid or unknown signatures, your local keyring is out of sync.
Easiest fix: open the pearOS Welcome app and click Fix Pacman Keys. It resets /etc/pacman.d/gnupg, re-populates the keyring and re-signs the pearOS archive key in one go (one password prompt).
Manual fix, if you'd rather use a terminal:
sudo rm -rf /etc/pacman.d/gnupg
sudo pacman-key --init
sudo pacman-key --populate archlinux cachyos pearos
sudo pacman -Syy
Still stuck? Let us know on Discord with the exact error pacman gives you.