This is what happens when I try to install ttf-pfc:

:: Proceed with installation? [Y/n]
:: Retrieving packages...
ttf-font-awesome-5.... 166.2 KiB 542K/s 00:00 [############] 100%
(35/35) checking keys in keyring [############] 100%
(35/35) checking package integrity [############] 100%
error: ttf-pfc: signature from "Márcio Silva <coadde@hyperbola.info>" is marginal trust
:: File /var/cache/pacman/pkg/ttf-pfc-20150920-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.

I recall that I have been at the devs numerous times about this but I fail to find a bug report about it when searching for it here.

I have tried to do most things when I first encountered this. "pacman-key --refresh-keys" is tried as well as "pacman-key --updatedb". If I remember correctly I have also tried to flush the entire PGP database that Pacman uses without anything changing in terms of success.