Add PDF writer.

test/publishing/TestPdfWriter.cpp

@@ -0,0 +1,18 @@
+#include "PdfDocument.h"
+#include "PdfWriter.h"
+#include "PdfObject.h"
+#include "File.h"
+
+int main()
+{
+    auto document = std::make_unique<PdfDocument>();
+
+    PdfWriter writer;
+    const auto output = writer.ToString(document);
+
+    File pdf_file("TestPdfWriter.pdf");
+    pdf_file.SetAccessMode(File::AccessMode::Write);
+    pdf_file.Open();
+    pdf_file.WriteText(output);
+    return 0;
+}