public class HomeController : Controller
{
    private readonly IHostingEnvironment _hostingEnv;
    private readonly ApplicationDbContext _context;

    public HomeController(IHostingEnvironment hostingEnv,ApplicationDbContext context)
    {
        _hostingEnv = hostingEnv;
        _context = context;
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Index(EngineerVM engineerVM)
    {
        if (engineerVM.File != null)
        {
            //upload files to wwwroot
            var fileName = Path.GetFileName(engineerVM.File.FileName);
            //judge if it is pdf file
            string ext =Path.GetExtension(engineerVM.File.FileName);
            if(ext.ToLower() != ".pdf")
            {
                return View();
            }
            var filePath = Path.Combine(_hostingEnv.WebRootPath, "images", fileName);

            using (var fileSteam = new FileStream(filePath, FileMode.Create))
            {
                await engineerVM.File.CopyToAsync(fileSteam);
            }
            //your logic to save filePath to database, for example

            Engineer engineer = new Engineer();
            engineer.Name = engineerVM.Name;
            engineer.FilePath = filePath;

            _context.Engineers.Add(engineer);
            await _context.SaveChangesAsync();
        }
        else
        {

        }
        return View();
    }
}