I want to List the documents that were attached during the initiation to the next approver (not as an email attachment but as a link on the task/approval form or flexi task form)
Call this web service Web_URL/_vti_bin/lists.asmx and use the GetAttachmentCollection method.
Parse the XML and you should have your url to the attachments. Will have to use a for each to get the individual attachment names as they should be stored in a collection variable.
Sorry but i am new to nintex. Do i call the web service in the form or the workflow ? and i'm using nintex office 365 not on premises
You can call the WebService in the workflow ("Call HTTP Web Service" action).
You have the documentation on how to use it in Office 365 here: Nintex Workflow for Office 365 - Calling a web service
collect task attachments in list item and show all attachments in next task might be helpfull.
Retrieving data ...