Folder results stores snapshot instead of pdf
Often when I'm looking at an embedded pdf in a Firefox window, the icon is a folder that when clicked gives only one choice. That choice has the title of the paper, but when I select it I get a snapshot, not a pdf. Full disclosure: I've only ever had this happen using SciHub, but it is the usual case for me with that site. The snapshot I get appears to come from the location of the paper from the SciHub metadata.
If you want to save the PDF directly, you'd have to open it in its own frame. (Generally you can do that by right-clicking on the frame, but I'm not sure if that's an option for a PDF in a frame.)
You can also save from the article page elsewhere and then attach a PDF manually.