Add a default page format constant

@@ -24,7 +24,7 @@ class PdfPage extends PdfObject {
   ///
   /// @param pdfDocument Document
   /// @param pageFormat [PdfPageFormat] describing the page size
-  PdfPage(PdfDocument pdfDocument, {this.pageFormat = PdfPageFormat.a4})
+  PdfPage(PdfDocument pdfDocument, {this.pageFormat = PdfPageFormat.standard})
       : super(pdfDocument, '/Page') {
     pdfDocument.pdfPageList.pages.add(this);
   }

@@ -39,6 +39,8 @@ class PdfPageFormat {
   static const PdfPageFormat legal =
       PdfPageFormat(8.5 * inch, 14.0 * inch, marginAll: inch);
 
+  static const PdfPageFormat standard = a4;
+
   static const double point = 1.0;
   static const double inch = 72.0;
   static const double cm = inch / 2.54;

@@ -64,7 +64,7 @@ typedef BuildListCallback = List<Widget> Function(Context context);
 
 class Page extends BasePage {
   Page(
-      {PdfPageFormat pageFormat = PdfPageFormat.a4,
+      {PdfPageFormat pageFormat = PdfPageFormat.standard,
       BuildCallback build,
       this.theme,
       EdgeInsets margin})