Once you've filled in some basic details, click "Find Match". Please note, you can switch between PubMed or Google Scholar using the drop-down menu in the bottom right.
If the correct article is found and listed, just double-click on it to match the PDF. This will be immediately updated in the desktop app and become available for citations.
Alternatively, if the PDF cannot be found in PubMed/Google Scholar, you can always enter metadata manually for any reference. Just right click on the PDF listing in your library and select "Edit reference". This brings up the metadata editing form. Just click the Save button in the bottom right when you're finished.