How to search for references with no attached full-text PDF

I have a large library where most of my references have a full-text PDF attached. How do I do a search, or define a Smart Group, to find me only those entries with no PDF attached?

You can create a Smart Group based on the search parameter: File Attachments + Is + [blank]. Refer to attached image.

Or, if there are file attachments of other types that you want to distinguish from PDFs, you  can search to omit those with pdf leaving only those that are blank or those that have attachments of other kinds.  

Thank you, that’s very helpful to know - that Endnote reads a blank field as a nil value and can run a search for it