Implement missing PdfDocument.Flatten() method

Author: mitchel

PdfSharpCore/Pdf/PdfDocument.cs

@@ -30,6 +30,7 @@
 using System;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using PdfSharpCore.Pdf.Advanced;
 using PdfSharpCore.Pdf.Internal;
 using PdfSharpCore.Pdf.IO;
@@ -797,6 +798,17 @@ public PdfPage InsertPage(int index, PdfPage page, AnnotationCopyingType annotat
             return Catalog.Pages.Insert(index, page, annotationCopying);
         }
 
+        /// <summary>
+        /// Flattens a document (make the fields non-editable).
+        /// </summary>
+        public void Flatten()
+        {
+            for (var i = 0; i < AcroForm?.Fields.Count(); i++)
+            {
+                AcroForm.Fields[i].ReadOnly = true;
+            }
+        }
+
         /// <summary>
         /// Gets the security handler.
         /// </summary>