For people who have done a lot of research in the past, and are now beginning to use Zotero, you may have numerous (hundreds!!) of .pdfs on your computer, which you would like to have in your Zotero account.
Zotero allows for the "drag and drop" of .pdfs from anywhere on your computer, into your Zotero library (some information here on the Zotero website).
When you drag a .pdf into your Zotero library, Zotero will attempt to retrieve metadata about the .pdf, and populate all the citation information for that article / book chapter etc. However, there may be times when Zotero will be unable to do this, and you will need to manually enter the citation information for that .pdf.