Adding a Downloaded PDF
Zotero allows you to import a PDF right into a collection. This will create a citation for the PDF.
To do this, first go into the Zotero collection you want the article to be imported into. Click the icon of the page with the plus sign and the bar, then click Add File. Choose the PDF from your files. It will be immediately added to the collection.
While this is a very handy function, you do need to check the citation when this process is complete. Imported PDFs may have incomplete information, such as missing authors, page numbers, etc.