Downloading attachments through API with URLs fails

When requesting to download an object, I get a "timing limited" URL of the form:

https://s3.amazonaws.com/SmartsheetB1/*******?response-content-disposition=attachment;filename="*************************.pdf";filename*=UTF-8''*************************.pdf&Signature=*********=&Expires=********&AWSAccessKeyId=***************

Trying this URL directly gives an error :

<?xml version="1.0" encoding="UTF-8"?>
InvalidRequestRequest specific response headers cannot be used for anonymous GET requests.********************************************************************************

I tried various pieces of the URL, but always end up with either Access Denied or "Query-string authentication requires the Signature, Expires and AWSAccessKeyId parameters",

even though they do exist.

I am using version 2.0 of the API.

Any help is appreciated.

Thank you