Fix page number order.

For case when root nod has reference to page the order will be incorrect.
The case if root node has reference
[2 0 R 3 0 R 10 0 R]
Where 2 0 R is intermediate node containing page 1,2,3

Where 3 0 R is intermediate node containing page 4,5,6
Where 10 0 R is page 7

without that fix 7 page will in array as page 1

src/UglyToad.PdfPig/Content/PageTreeNode.cs

@@ -32,7 +32,7 @@
         /// <summary>
         /// The number of this page if <see cref="IsPage"/> is <see langword="true"/>.
         /// </summary>
-        public int? PageNumber { get; }
+        public int? PageNumber { get; internal set; }
 
         /// <summary>
         /// The child nodes of this node if <see cref="IsPage"/> is <see langword="false" />

src/UglyToad.PdfPig/Parser/CatalogFactory.cs

@@ -5,8 +5,10 @@
     using Content;
     using Core;
     using Parts;
+    using System.Linq;
     using Tokenization.Scanner;
     using Tokens;
+    using Util;
 
     internal static class CatalogFactory
     {
@@ -81,6 +83,8 @@
                 return new PageTreeNode(nodeDictionaryInput, referenceInput, true, pageNumber.PageCount).WithChildren(EmptyArray<PageTreeNode>.Instance);
             }
             
+            
+
             //If we got here, we have to iterate till we manage to exit
 
             var toProcess =
@@ -126,8 +130,6 @@
 
                     if (isChildPage)
                     {
-                        pageNumber.Increment();
-
                         var kidPageNode =
                             new PageTreeNode(kidDictionaryToken, kidRef.Data, true, pageNumber.PageCount).WithChildren(EmptyArray<PageTreeNode>.Instance);
                         current.nodeChildren.Add(kidPageNode);
@@ -152,6 +154,12 @@
                 action();
             }
 
+            foreach (var child in firstPage.Children.ToRecursiveOrderList(x=>x.Children).Where(child => child.IsPage))
+            {
+                pageNumber.Increment();
+                child.PageNumber = pageNumber.PageCount;
+            }
+
             return firstPage;
         }
 

src/UglyToad.PdfPig/Util/EnumerableExtensions.cs

@@ -0,0 +1,44 @@
+namespace UglyToad.PdfPig.Util
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Linq.Expressions;
+    using System.Reflection;
+
+
+    internal static class EnumerableExtensions
+    {
+        public static List<T> ToRecursiveOrderList<T>(this IEnumerable<T> collection,
+            Expression<Func<T, IEnumerable<T>>> childCollection)
+        {
+            var resultList = new List<T>();
+            var currentItems = new Queue<(int Index, T Item, int Depth)>(collection.Select(i => (0, i, 0)));
+            var depthItemCounter = 0;
+            var previousItemDepth = 0;
+            var childProperty = (PropertyInfo)((MemberExpression)childCollection.Body).Member;
+            while (currentItems.Count > 0)
+            {
+                var currentItem = currentItems.Dequeue();
+                // Reset counter for number of items at this depth when the depth changes.
+                if (currentItem.Depth != previousItemDepth)
+                {
+                    depthItemCounter = 0;
+                }
+
+                var resultIndex = currentItem.Index + depthItemCounter++;
+                resultList.Insert(resultIndex, currentItem.Item);
+
+                var childItems = childProperty.GetValue(currentItem.Item) as IEnumerable<T> ?? Enumerable.Empty<T>();
+                foreach (var childItem in childItems)
+                {
+                    currentItems.Enqueue((resultIndex + 1, childItem, currentItem.Depth + 1));
+                }
+
+                previousItemDepth = currentItem.Depth;
+            }
+
+            return resultList;
+        }
+    }
+}