BUG: Updated pdf fields don't show up when page is written (#412)

Closes #355
See #300

Borrowed code from ademidun in the comment history and inserted it into the
proper location in the pdf.py module.

Co-authored-by: Tomiwa Ademidun <[email protected]>

PyPDF2/pdf.py

@@ -118,6 +118,7 @@ def __init__(self):
             })
         self._root = None
         self._root_object = root
+        self.set_need_appearances_writer()
 
     def _addObject(self, obj):
         self._objects.append(obj)
@@ -136,6 +137,23 @@ def _addPage(self, page, action):
         action(pages[PA.KIDS], page)
         pages[NameObject(PA.COUNT)] = NumberObject(pages[PA.COUNT] + 1)
 
+    def set_need_appearances_writer(self):
+        # See 12.7.2 and 7.7.2 for more information:
+        # http://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf
+        try:
+            catalog = self._root_object
+            # get the AcroForm tree
+            if "/AcroForm" not in catalog:
+                self._root_object.update({
+                    NameObject("/AcroForm"): IndirectObject(len(self._objects),
+                        0, self)})
+
+            need_appearances = NameObject("/NeedAppearances")
+            self._root_object["/AcroForm"][need_appearances] = BooleanObject(True)
+
+        except Exception as e:
+            print('set_need_appearances_writer() catch : ', repr(e))
+
     def addPage(self, page):
         """
         Adds a page to this PDF file.  The page is usually acquired from a