I made a key pair pem file called "test.pem", and I downloaded to my PC.
I made a new instance with this pem file.
Now I am in a different pc, and I don't have this pem file in my local, and my previous pc is in the middle of the sea (shipping).
How can I re-download the "test.pem" file again?
No, you cannot download .pem file again. You can download the .pem file ONLY once and that is when you create a new key-pair.