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this button allows you to create manually new references if you can not find the reference online. To choose the type of the material just click the arrow next to it and it shows different options. Choose the type according yours.
Add References of PDFs
Save the PDFs you would like to put into your library on your computer, then drag and drop them into your Zotero library. Zotero will automatically attempt to retrieve metadata (title, author, journal...) from the PDF. The PDF is also renamed according to parent metadata.
Add a Reference Through the ISBN or DOI
this button allows you to add references using the ISBN (International Standard Book Number) or the DOI (Digital Object Identifier). All books which are being published nowadays have an ISBN. The DOI is an identifier for electronic documents such as Web pages, or articles published in electronic journals, but most Web pages do not have DOIs. But you can find DOIs in some bibliographic databases.
Using Zotero Connector / Using the Icon in the URL Bar
Zotero uses "translators" to convert the data from library catalogs, databases, or other Web sites such as Google or Amazon. When you are viewing a Web site, if a translator is available, a little icon appears in the URL bar. Just click on it, and the reference is downloaded.
There are different icons, for each kind of documents: blue for books , white for journal articles ... If there are several references on one Web page, the folder icon appears, and if you click on it, you can select which references you wish to download.