You'd see "Zotero is disabled" when viewing a PDF if you have some other browser extension installed (e.g., Acrobat) that's intercepting the PDF (with an extension:// or similar URL) or if you're viewing a local file (with a file:// URL). Zotero can only save from http:// and https:// URLs.
Make sure your browser is set to preview PDFs — so they show at http(s):// URLs — rather than download them, and uninstall any browser extension that might intercept PDFs.
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Make sure your browser is set to preview PDFs — so they show at http(s):// URLs — rather than download them, and uninstall any browser extension that might intercept PDFs.