How to search for records where a field doesn't exist

If this has been answered in another thread, please link it here. It could be described in different ways by different people and I was not able to think of any unique keywords or phrases that would return relevant answers from the forums.

Does anyone know how to pull up records in a search based on a field not existing? I would like to have a saved search that contains journal citations that don't have a PDF. When I search with "Attachment File Type" is not "PDF", it pulls up everything with at least one attachment that isn't a PDF, which is pretty much everything.
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