FileController.cs

[Route("File")]
public class FileController : Controller
{
    [HttpPost("Download/{id:guid}")]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Download(Guid id)
    {
        // Grab a test.pdf back one directory.  Look ma - it even runs on Linux with Path.DirectorySeparatorChar!
        var fileContent = await File.ReadAllBytesAsync($"..{Path.DirectorySeparatorChar}test.pdf");
        
        return File(fileContent, "application/pdf")
    }
}