When you click/tap on a link for a PDF file in Microsoft Edge, the PDF file will open in Microsoft Edge using the internal PDF viewer by default.
If you turn off Always open PDF files externally, Microsoft Edge treats PDF files as downloads and lets users open them with the default app instead.
This tutorial will show you how to enable or disable always open PDF files externally in the Chromium based Microsoft Edge.
EXAMPLE: "Always open PDF files externally" turned on or off in Microsoft Edge
Read more: Browsers and Mail - How to Enable or Disable Open PDF files in Microsoft Edge | Windows Questions