When articles import into Papers they are automatically matched to the metadata so you know exactly what the article is. Sometimes, you may need to rematch metadata, which you can easily do by right-clicking the reference. You will see the different options below:
Edit Details will bring up the metadata editing pane, including the PDF preview so you can easily copy the fields to populate the reference.
Clear Details will erase all metadata from the reference. The name will revert back to the original download name. From here you can repopulate all the metadata fields or use Lookup Details to find a match.
Selecting Update Details will rerun the matcher and populate any missing metadata fields.