Make all files in S3 Bucket readable by public

There is no specific setting to allow this, instead you need to Edit Bucket policy from within the console:

Create file

{
    "Version":"2012-10-17",
    "Statement":[
        {
            "Sid":"AddPerm",
            "Effect":"Allow",
            "Principal": "*",
            "Action":["s3:GetObject"],
            "Resource":["arn:aws:s3:::examplebucket/*"]
        }
    ]
}

Upload file

more examples here