I frequently want to import PDFs that I download from some websites into paperwork. Normally opening a PDF with paperwork imports it, but unfortunately firefox has a bug which makes “opening with paperwork” in the download dialog not work correctly: 1758290 - Cannot open file with app with a desktop files specifying options in Exec=
There is a reasonably nice workaround for that: install junction and parametrize firefox to always open PDFs with junction. Junction is not affected by the same bug as firefox, so when you tell junction to open the PDF with paperwork, it will properly import the PDF into paperwork. This way, when you download a PDF, you will be greeted with a dialog to choose what application to open the PDF with, and you can choose either paperwork or your favorite PDF reader 
I hope you will find this useful!