AWS EC2 - Permission denied

Here’s what I’m entering in Terminal on OS X:

chmod 400 ~/.ssh/adamcarter-key-pair.pem
ssh -i ~/.ssh/adamcarter-key-pair.pem ec2-user@52.11.35.5

And am getting this in return:

Permission denied (publickey).

I’ve tried chmod 600, putting the .pem file in my Downloads folder, using all of the alternatives to ec2-user in the docs. I have no idea what else I can try. I’ve also tried the longer address under ‘Public DNS’.

One thing I did notice is when I created my key pair, it was instantly downloaded as a .txt file. I’ve changed the extension to .pem though so that shouldn’t be a problem, should it?

Any ideas would be very welcome!