Hi @Denny_Chan 

Would probably need to know a little more about the use case and what you would like to solve but for me the best way to merge forms and documents would be to use a document set content type, these essentially work as folders in a Document library but have some key advantages, one of which being the ability to have a Nintex form on the document set its self.

Hope this helps.

Kind regards

Hi @Denny_Chan ,

The File upload control within Nintex forms for Office 365, only works with Items attachments and not Document. By design, in modern UI documents does not have attachments. 

If you need to group documents together, @Jake @is right, you should use Document Set features which is a specific content type grouping documents with common metadata and/or specific between documents. 

You could use hybrid solution and use Nintex Workflow Cloud solution if you need from the same form submit metadata and all documents to regroup together within folder and/or document set. You will be able to create the document set and/or folder (if not already there) and upload all documents automatically.

Hope that helps. ^^

Cecilia