Refactor document handling and improve file response

Refactored the handling of `senderDoc.ByteData` by replacing
the ternary operator with an explicit `if` statement for better
readability. Added a `Content-Disposition` header to ensure
the file is displayed inline with a proper filename. Updated
the MIME type of the file response from `application/octet-stream`
to `application/pdf` to reflect the expected content type.

EnvelopeGenerator.API/Controllers/DocumentController.cs

@@ -87,8 +87,10 @@ public class DocumentController(IMediator mediator, IAuthorizationService authSe
 
         var senderDoc = await mediator.Send(new ReadDocumentQuery() { EnvelopeId = envelopeId }, cancel);
 
-        return senderDoc.ByteData is byte[] senderDocByte
-            ? File(senderDocByte, "application/octet-stream")
-            : NotFound("Document is empty.");
+        if (senderDoc.ByteData is not byte[] senderDocByte)
+            return NotFound("Document is empty.");
+
+        Response.Headers.ContentDisposition = $"inline; filename=\"{envelopeKey}.pdf\"";
+        return File(senderDocByte, "application/pdf");
     }
-}
+}