How do you do download the files folder from the Acquia cloud service?
You can read up on some drush specific tasks that you can do in this Elevated Third blog post 

Acquia supplies drush commands and aliases that tie into Acquia Cloud API calls. So you can actually simply use the alias and the rsync drush command:

drush rsync --progress @sitename.env:%files @localenv:%files


You can also create and download Acquia database backups with the ac-database-instance-backup and ac-database-instance-backup-download commands.

Also, if you want an "all-in-one" solution, the newer versions of Acquia DevDesktop tie directly to your Acquia cloud account and can get database and file syncs automatically.