The application (or browser functionality) showing you the PDF should offer a button for that. However, the website might customize or replace that application to prevent it to give you a download button.
But, those functionalities (PDF viewing) are mostly in the client, so you need the PDF file locally anyway, so you might find it in the Network tab in the web developer tools (F12).
Yes, I misremembered/misread the original text. Thank you for noticing me.