You can use this method to download files from CRM to your system.
https://crm.zoho.com/crm/private/xml/Leads/downloadFile?authtoken=Auth Token&scope=crmapi&id=Attachment Id
https://crm.zoho.com/crm/private/json/Leads/downloadFile?authtoken=Auth Token&scope=crmapi&id=Attachment Id
|authtoken*||String||Encrypted alphanumeric string to authenticate your Zoho credentials.|
|scope*||String||Specify the value as crmapi|
|id*||String||Specify unique ID of the attachment|
* - Mandatory parameter