We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
How do I import references into Zotero?
Zotero's “Import” function imports citation data from bibliographic formats such as bibtex or RIS. To get files like PDFs into Zotero, simply drag&drop them to the Zotero interface or use “Store Copy of File”. See here for more information on getting files into Zotero.
For information on using Zotero's import functions to add items to your library, see the main documentation.
For documentation on the data formats that Zotero supports, see the first link above, or the more detailed descriptions in Zotero's developer documentation.