Files (or links to files) can be added to your Zotero library. These files can either be standalone items, or attached to a reference as a child item. These files will count towards the 300MB of free storage space that Zotero provides for its users. If you choose to add the file as a link, the file will not take up space in your Zotero account, but will instead link to the file saved in your computer.
To add an attachment, you can drag a file from your system's file browser and drop it into the center Zotero pane, a collection on the left pane, or into a reference item. The last option will create a child item.
Another way to add an attachment is to add it directly from Zotero.
To add a child item, select a reference item, click on the "Add Attachment" paperclip button, and select either Attach Stored Copy of File... or Attach Link to File... and select the file you want to add.
To add a standalone item, click on the "New Item" button, and select either Store Copy of File or Link to File... and select the file you want to add.
Note: It is recommended to always add files as child files, as standalone files cannot be used with many of Zotero's features, including citing, because they do not contain any bibliographic data.