Microsoft Edge definitely acts as a default PDF viewer for the documents but remains as default even when any other PDF reader like Acrobat reader is installed. It means it will automatically open the PDF documents. This newly introduced browser Microsoft Edge is the default PDF viewer as well in system with Windows 10.
It is just same as the Internet Explorer comes as a default web browser with earlier versions of Windows 7 or 8. MS Edge comes as a default web browser when you install Windows 10.
