I found that i needed to mention the font style that i have added previously on this

doc.addFileToVFS("Amiri-Regular.ttf", arabicFont);
doc.addFont("Amiri-Regular.ttf", "Amiri", "normal");
doc.setFont("Amiri");

so my headstyles should look like this

headStyles: { font: "Amiri", fontStyle: "normal" }