CloudFront is an Amazon Web Services (AWS) Content Delivery Network (CDN) service. One option is to return objects (files) in an S3 Bucket. For example, let's say you have an index.html file in your S3 Bucket.

Let's create a CloudFront Distribution that uses the S3 Bucket as the origin.

By default, S3 Buckets are set to block all public access. 

Assuming your S3 Bucket is configured to block all public access, let's also go with "Origin access control settings (recommended)" so that we can keep the S3 Bucket configured to block all public access

After creating the CloudFront Distribution, there should be a banner. Let's Copy policy.

The policy JSON will be something like this.

{
    "Version": "2008-10-17",
    "Id": "PolicyForCloudFrontPrivateContent",
    "Statement": [
        {
            "Sid": "AllowCloudFrontServicePrincipal",
            "Effect": "Allow",
            "Principal": {
                "Service": "cloudfront.amazonaws.com"
            },
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::my-bucket-abc123/*",
            "Condition": {
                "StringEquals": {
                    "AWS:SourceArn": "arn:aws:cloudfront::123456789012:distribution/E3P9XL2JIBYIF"
                }
            }
        }
    ]
}

Let's update the S3 Bucket Policy to have the JSON.

You should now have a Distribution domain name (d25ajrhgrwb9yb.cloudfront.net in this example). It may take a while, perhaps about 5 minutes, for the Distribution to be ready. Once Last modified shows a date and time, the Distribution should be good to go.

And there should be a behavior that contains HTTP and HTTPS for your S3 Bucket.

And you should be able to return index.html at d25ajrhgrwb9yb.cloudfront.net/index.html.