How to get Document Viewer Field working

This will not work with newly-uploaded documents, until the form they're uploaded on is submitted. 

Until that point, they exist in a special "quarantine" area in the Appian filesystem, where they're not accessible by conventional means (including as targets of the "document()" function, and the preview field).  Such uploads are only moved to their final "target" location after form submission.

First, are you trying to display the document on the same interface as it is being uploaded on, or is it uploaded and displayed later / on a separate interface?

Just to note that until a document is uploaded by submitting the form, it is essentially a temporary object and not an official 'document' at this point, in case this is the situation.