Zotero can automatically attach and store associated pdfs and other files when saving references.  To confirm that it is set up to do this, go to Edit, then Preferences.

Checkmark the areas below (if not already marked by default).

Starting from an example of a full text article in a library database.

Any associated file should appear within your Zotero library or collection.

You can drag and drop pdfs into your collections or library.

If the pdf has been OCR'ed (Optical Character Recognition) then the item's text, such as metadata (title, author, date, etc.) will be accessed automatically to create the citation information.  If the pdf is not OCR'ed, you may enter the citation information manually.