It depends. If the site allows viewing the PDF in Firefox's built-in PDF viewer, there is a download button near the right end of the PDF-specific toolbar. If you want to test with a basic PDF, here's one from the U.S. copyright office: https://copyright.gov/ai/ai_policy_guidance.pdf You should see a folder with a down arrow on the Save As button. Command+S (Save Page As) should also work. If the site forces you to view the PDF in their own viewer, then the site control whether a download button is available. This can be tricky to work around.