There are some general problems saving PDFs from ScienceDirect that we're working on, but you should save from the article page, not the PDF, on ScienceDirect and all other supported sites.
Do you not see a PDF listed in the save dialog, or do you see the PDF listed with a red X?
If the latter, it's likely just the problem described in the linked thread. If you have Zotero 7, you should generally see a CAPTCHA dialog in Zotero (not your browser) when you try to save from ScienceDirect.
If the latter, it's likely just the problem described in the linked thread. If you have Zotero 7, you should generally see a CAPTCHA dialog in Zotero (not your browser) when you try to save from ScienceDirect.