Write greek language Rmarkdown pdf or html document

I really need to write a Rmarkdown pdf document, I tried this in english version

 ---
title: "Report"
subtitle: "Company S.A."
output:
 bookdown::pdf_document2:
toc: true
number_sections: true
header-includes:
 - \usepackage{titling}
 - \pretitle{\begin{center}
   \includegraphics[width=2in,height=2in]{logo.png}\LARGE\\}
 - \posttitle{\end{center}}
---

and works just fine I have the pdf document that I want. When I try to write the same exact document in greek, It does't work anything it cannot recognize the greek language. so I tried this, but also did not work.

 header_includes:
    - \usepackage[utf8x]{inputenc}
    - \usepackage[english,greek]{babel}
    - \newcommand{\en}{\selectlanguage{english}}
    - \newcommand{\gr}{\selectlanguage{greek}}
 title: "Φύλλο αναφοράς"
subtitle: "Εταρεία Α.Ε."
output:
bookdown::pdf_document2:
toc: true
number_sections: true
---

\gr Καλημερα

Does anyone know how tosolve this problem? I need two documents to write in greek one in html and one in pdf.

using this code

---
title: "\\gr Φύλλο αναφοράς"
subtitle: "\\gr Εταρεία Α.Ε."
 output:
 bookdown::pdf_document2:
toc: true
number_sections: true
keep_tex: true
header-includes:
 - \usepackage[english,greek]{babel}
 - \newcommand{\en}{\selectlanguage{english}}
 - \newcommand{\gr}{\selectlanguage{greek}}
---

 \gr Καλημερα

it gives me this error

while using this code

---
 title: "Φύλλο αναφοράς"
 subtitle: "Εταρεία Α.Ε."
  output:
   bookdown::pdf_document2:
   toc: true
    number_sections: true
    keep_tex: true
    latex_engine: xelatex
    header-includes:
     - \usepackage{fontspec}
   - \setmainfont{TeX Gyre Heros}
   ---

   Καλημερα

it gives me this error