Customer.cs

[Route("api/[controller]")]
[ApiController]
public class ViewerController : ControllerBase
{
  [HttpGet]
  [Route("LoadDocument")]
  public LoadedDocument LoadDocument()
  {
    string documentName = "invoice.docx";

    LoadedDocument document = new LoadedDocument()
    {
      DocumentData = Convert.ToBase64String(
        System.IO.File.ReadAllBytes("App_Data/" + documentName)),
      DocumentName = documentName
    };

    return document;
  }
}