drop copies files into a Notebook – is it possible to create a link to a file instead?
Yes. To create a link, drag a document from the Finder over your Notebook, and then press the Control key. When you do, the cursor will change into the “link” cursor (you may have to jiggle the mouse a little to get it to appear). When you release the document, NoteBook creates a link to the original. Linked attachments look just like regular attachments that have been copied into a Notebook, except for a little link arrow in the lower-left corner of the icon/image. You can make linking the default action through the Drag-and-Drop section in NoteBook’s Preferences panel.