By defaut PDF documents are opening within 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)
Work-around to open PDF files outside browser with Adobe application:
Edge: Type in address bar edge://settings/content/pdfDocuments then press Enter and select "Always Download PDF files"
FireFox: Type in address bar about:preferences#general then press Enter, scroll down to Applications and select for Adobe Acrobat Document to open with your Windows Default Program for PDF files ie. Adobe Acrobat or Reader
Chrome: Type in address bar chrome://settings/content/pdfDocuments then press Enter and select "Always Download PDF files"