Download pdf where? You provide no URL or screenshot of the issue for anyone reading this to know what or where to see what is happening.

Firefox recently did a software update and I can no longer download pdfs. Can you help me?

Are you getting an error message?

Does it make any difference if you experiment with different settings for the action Firefox should taken when downloading a PDF: View PDF files in Firefox or choose another viewer?

No error message except it simply says in the downloads that the download failed. It doesn't make a difference to change the settings for action when downloading. Any help to this?