Either place the file in wwwroot or write an action/handler to return a file. Also see your other thread with the same subject.
Path is the absolute path to the file. Keep in mind, Razor Pages are compiled into a binary.
public async Task<IActionResult> OnPostDownloadFile()
{
Byte[] buffer = await System.IO.File.ReadAllBytesAsync(path);
return File(buffer, "text/html");
}
The editor that opens the file depends on what applications are installed on the client's machine and how the default settings are configured.