I still drag and drop PDFs in Zotero 7. It's less finicky then v6, even. The trick is to drop them to root of the tree on the left. Not to the middle of the interface.

Drag-and-drop of the PDF file worked for me in Zotero 9 (latest version). I never used this feature before. This would be greatly preferred to my earlier suggestion to get a LLM to generate a list of DOIs.

I drag-drop PDFs and ePubs, read PDFs and ePubs, and sync mobile with laptop, etc.