If your using Edge with ePub you are probably using Edge classic (AFAIK new edge does NOT support ePub, without extensions) Classic should also have support for PDF, that was why it became a PITA as it stopped SumatraPDF being default PDF reader.
Launching a pdf into new edge is as easy as calling edge.exe with the filename and optionally page number to open at. However I use other edge features so dont pass the page number. one of my ExternalViewers is Alt+F+D
CommandLine = "C:\Program Files (x86)\Microsoft\Edge\Application\msedge.exe" -inprivate -profile-directory=Default "%1"
Name = e&Dgium [Gif movie] | [Cast To / Inking / Print / Read Aloud]
Filter = *.gif;*.pdf;*.txt