What is the collective noun for a stack of paper?

There isn't a specific collective noun for a stack of paper. However, depending on the context, you could use:

* Pile

* Heap

* Stack (which is already a collective noun but can be used as a more general term)

* Ream (specifically for a bundle of 500 sheets)

You could also use more descriptive phrases like:

* Tower of paper

* Mountain of paper

* Bundle of documents