How do I have Safari (v12) auto download PDF files
The Acrobat/Reader plug-in for Apple Safari is based on Netscape Plug-In API (NPAPI), which is supported only till Safari version 11. In Safari 12, support for NPAPI plug-ins is removed, and hence the current Acrobat/Reader plug-in will not work. For detailed information, please refer to help articles Display PDF in browser Adobe Acrobat, Acrobat Reader

What’s New in Safari - Apple Developer

Source: Adobe.com