Increase PDF version to 1.7

@@ -2,6 +2,7 @@
 
 * Add Document ID
 * Add encryption support
+* Increase PDF version to 1.7
 
 ## 1.3.13
 

@@ -43,6 +43,9 @@ class PdfCatalog extends PdfObject {
   void _prepare() {
     super._prepare();
 
+    /// the PDF specification version, overrides the header version starting from 1.4
+    params['/Version'] = PdfStream.string('/${pdfDocument.version}');
+
     params['/Pages'] = pdfPageList.ref();
 
     // the Outlines object

@@ -89,6 +89,9 @@ class PdfDocument {
   /// Object used to encrypt the document
   PdfEncryption encryption;
 
+  /// The PDF specification version
+  final String version = '1.7';
+
   /// These map the page modes just defined to the pagemodes setting of Pdf.
   static const List<String> _PdfPageModes = <String>[
     '/UseNone',