Another vote for negative searches from me. I've just imported a Zotero library containing, among others, all my personal papers, abstracts, letters, etc. They have article type as "journal article" but with tags to distinguish letters, abstracts, etc. Sadly, I have no specific tag for peer reviewed papers, since in Zotero these could easily be listed with a negative search e.g. "not abstract".
Sadly, I am a bit stuck to do the same in Paperpile I suppose I can delete my whole Paperpile library and reimport with appropriate tags but I really ought to be getting back to some proper work . . .