it would be great to be able to create a folder that contains all the papers cited in a particular document. Ideally, PP would have another category - call it DOCUMENTS, perhaps. Then PP would auto-create folders within the category, with the name the same as the document title. Then the DOCUMENT-folder would contain all papers cited therein.
2nd best implementation would be done manually/static, by having a button within the PP menu - called ‘add cited papers to DOCUMENTS’
If you dont want to / cannot add a new category, then add the papers to either folders or labels.