Getting 403 forbidden from s3 when attempting to download a file

I think you need to make sure that the permissions are applied to objects when moving/copying them between buckets with the "bucket-owner-full-control" acl.

Here are the details about how to do this when moving or copying files as well as retroactively: https://aws.amazon.com/premiumsupport/knowledge-center/s3-bucket-owner-access/

Also, you can read about the various predefined grants here: http://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html#canned-acl

To correctly set the appropriate permissions for newly added files, add this bucket policy:

[...]
{
    "Effect": "Allow",
    "Principal": {
        "AWS": "arn:aws:iam::123456789012::user/their-user"
    },
    "Action": [
        "s3:PutObject",
        "s3:PutObjectAcl"
    ],
    "Resource": "arn:aws:s3:::my-bucket/*"
}

Your bucket policy is even more open, so that's not what's blocking you.

However, the uploader needs to set the ACL for newly created files. Python example:

import boto3

client = boto3.client('s3')
local_file_path = '/home/me/data.csv'
bucket_name = 'my-bucket'
bucket_file_path = 'exports/data.csv'
client.upload_file(
    local_file_path,
    bucket_name, 
    bucket_file_path, 
    ExtraArgs={'ACL':'bucket-owner-full-control'}
)

source: https://medium.com/artificial-industry/how-to-download-files-that-others-put-in-your-aws-s3-bucket-2269e20ed041 (disclaimer: written by me)

The problem here stems from how you get the files into the bucket. Specifically the credentials you have and/or privileges you grant at the time of upload. I ran into a similar permissions issue issue when I had multiple AWS accounts, even though my bucket policy was quite open (as yours is here). I had accidentally used credentials from one account (call it A1) when uploading to a bucket owned by a different account (A2). Because of this A1 kept the permissions on the object and the bucket owner did not get them. There are at least 3 possible ways to fix this in this scenario at time of upload:

• Switch accounts. Run $export AWS_DEFAULT_PROFILE=A2 or, for a more permanent change, go modify ~/.aws/credentials and ~/.aws/config to move the correct credentials and configuration under [default]. Then re-upload.
• Specify the other profile at time of upload: aws s3 cp foo s3://mybucket --profile A2
• Open up the permissions to bucket owner (doesn't require changing profiles): aws s3 cp foo s3://mybucket --acl bucket-owner-full-control

Note that the first two ways involve having a separate AWS profile. If you want to keep two sets of account credentials available to you, this is the way to go. You can set up a profile with your keys, region etc by doing aws configure --profile Foo. See here for more info on Named Profiles.

There are also slightly more involved ways to do this retroactively (post upload) which you can read about here.