Make sure you have installed the Zotero Connector.
Once installed, the connector will appear as an icon in your browser toolbar.

To add a source to Zotero, click on the Zotero icon in your browser toolbar.

Zotero will save relevant information about the source into your Zotero Library, such as:
This information will help you organize and access the source at a later time and provide citation information.
How to add a PDF (e.g. article) from your computer to a reference in Zotero
OR
Manually add an item to your Zotero library:
) button at the top of the center column.