It would be nice to be able to exclude labels, folders, or filters from specific search results. For instance, when searching for starred books I would like to exclude items I have already read. Or, when looking for journal articles about a specific subject, I would like to exclude items which are in my “teaching” folder. Etc.
Perhaps this is already possible, but there is not a clear interface or instructions for it?
No unfortunately it’s not possible at the moment.
Can I also upvote this suggestion? And in general - more robust document sorting. A lot of the utility of a reference manager for me is to be able to put read papers into shelves - which means not having an “unsorted” folder leads me to resort to using other ref. managers for my organization needs.
A filter for ‘unsorted’ items is planned and will definitely be added in a future update.
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 . . .
Why not just label all peer reviewed items within Paperpile? I don’t see the need to re-import from Zotero.
If you already have tags for the different types and they got imported into Paperpile it should be easy to select e.g. ‘Research article’ or whatever your Zotero tag is and add a label 'Peer reviewed".
It seems @srunderwood did not have any tag in Zotero, but relied on a negative search. “NOT” searches are a standard Boolean operator and it is not unusual to incorporate it into one’s workflow. For instance, I would love to be able to search for items with the tag TAIWAN that are not tagged INDIGENOUS. Many of my papers are about indigenous Taiwanese, but sometimes I’m looking for something that is about Taiwan but which is not related to indigenous issues. Excluding them from the search would save me a lot of time.
Some background. It’s not that we don’t see the use of this feature or don’t like it. It’s more of a technical issue how our database filter works. Our goal is to show results within 200ms (+/- depending on your geographic location). This requires some tricks to search efficiently through an ever growing database of millions of items. It’s not easy to do that with OR or NOT. So these seemingly simple features are sometimes the hardest to implement.
In the same vein, it would be really nice to be able to search for unlabelled papers.
Up vote this from me too.
Why not just create two groups for each option, a positive and a negative one? You seem to see this value and do this in the use of PDF and no PDF. In fact, under that construct of every item is in or out of every group, then computation in some ways is easier because for every tag or folder all items always have a value (positive or negative). Then it just becomes an interface issue, which can be done quite well with simple things (e.g. I note that you already allow a multi-select of folders with the shift key - same could be done with filters turning them on or off with toggle - or shift). Hope I’ve been clear and helpful and not pedantic. In any case, I’d love to have the feature.
Is this still on? Is there any plan to incorporate this new feature?
Thanks for the bump here, @Marc-Andre_Vigneault! This is definitely still on - actually, improving library searches is one of our current priorities. We are in the process of testing this and other new features like full-text search, phrase search and specific author/date searches, so it hopefully it won’t be long before we have some news to share.
Another bump!
Is this still not possible?
Having this feature would be a real timesaver!
@Jaik this will be possible soon – advanced search features are part of the upcoming renovation of our web-app and extension (as posted on our roadmap). First private beta is expected within the next couple months and wider release before the end of this year 
Another upvote from me! You’ve got AND condition working (I figured out it works with picking two on more with shift pressed) so adding NOT (and OR!) could be a natural extension and a great feature for those who use labels more extensively than folders.