When you upload a document to a K2 for SharePoint - integrated document library in SharePoint via the New Document link, you do not see a K2 SmartForm.
To illustrate the difference between how a SharePoint list and SharePoint document library function, consider how new items are added to each list/library.
When adding a new item to a SharePoint list, the (+) new item button at the top of the list maps to the New Item button on the ribbon.
When adding a new document to a SharePoint library, the (+) new document button at the top of the library maps to the Upload Document ribbon button, not New Document.
This is by design. K2 for SharePoint uses the same UI functionality as SharePoint. This is because the SharePoint API does not provide the same functionality for the New Document option in a document library as it does for the New Item option in a list. The auto-generated New Document SmartForm is not used when uploading a document to a SharePoint library as the SharePoint New Document form is always used for Upload Document and cannot be customized by any app.
Note that clicking the New Document option in the SharePoint ribbon opens Microsoft Word.
The New Document SmartForm is available, as it is created when integrating the document library with K2 for SharePoint, and you can use it outside of SharePoint to upload documents to SharePoint.
If you add a custom column to the document library, the Edit Document SmartForm is mapped to the New Document link.