invalid or corrupted package (PGP signature):
Default number of Maximum Parallel Downloads after a fresh installation is set to 4. It prevents medium and large packages such as Inkscape, Libre Office, Audacity from being installed and prompts error "invalid or corrupted package (PGP signature):". It is only after setting the Maximum Parallel Downloads to 1, or deleting the below lines from file "/etc/pamac.conf" it goes back to normal:
## Maximum Parallel Downloads
MaxParallelDownloads = 4