References from databases (such as PubMed, Google Scholar, Scopus, and CINAHL) can be easily imported into Zotero via the Zotero Connector.
Steps for importing a reference into Zotero from a database:
To store content from the Web, simply click on the Zotero Connector located in the upper right corner of your browser (while on a website, it should look like a document). While this will save basic metadata (title, URL, access date), you may need to fill in additional metadata from the page by hand.
To include a PDF or any other file from your computer in Zotero, simply drag it to Zotero — you can drag to an existing item to attach the file or drag between items to store as an independent item. Zotero will automatically retrieve reference information for the PDFs.