By default PDF documents are opening within the browser.
The following browsers have dropped support for NPAPI, and therefore Acrobat/Acrobat Reader plug-in does not work on these browsers anymore: Microsoft Edge, Mozilla Firefox (v52 and above), Google Chrome (September 2015 release onwards), Apple Safari (v12 and above).
Workaround to open PDF files outside the browser with the Adobe application:
Edge: Type in the address bar edge://settings/content/pdfDocuments then press Enter and select Always Download PDF files.
FireFox: Type in the address bar about:preferences#general then press Enter, scroll down to Applications, and select Adobe Acrobat Document to open with your Windows Default Program for PDF files ie. Adobe Acrobat or Reader.
Chrome: Type in the address bar chrome://settings/content/pdfDocuments then press Enter and select Always Download PDF files.