Is it safe to assume that you're running the Invoke-Command from a machine that isn't PC1 or PC2?
If that's true you've encountered the "second hop" problem: ps-remoting-second-hop
If, on the other hand, you're running the Invoke-Command from PC1 then there's no need for the "-ComputerName PC1" -- but you still have the "second hop" problem because you cannot use the credentials from PC1 to access the shared directory on PC1 from PC2.