diff --git a/.vscode/settings.json b/.vscode/settings.json
new file mode 100644
index 0000000..b038473
--- /dev/null
+++ b/.vscode/settings.json
@@ -0,0 +1,5 @@
+{
+  "[pdf]": {
+    "files.trimTrailingWhitespace": false,
+  }
+}
diff --git a/lib/rubrik/document.rb b/lib/rubrik/document.rb
index f300bdb..e629bd1 100644
--- a/lib/rubrik/document.rb
+++ b/lib/rubrik/document.rb
@@ -17,19 +17,25 @@ class Document
     sig {returns(PDF::Reader::ObjectHash)}
     attr_accessor :objects
 
+    sig {returns(Integer)}
+    attr_accessor :first_free_object_id
+
     sig {returns(T::Array[{id: PDF::Reader::Reference, value: T.untyped}])}
     attr_accessor :modified_objects
 
     sig {returns(Integer)}
     attr_accessor :last_object_id
 
-    private :io=, :objects=, :modified_objects=, :last_object_id=
+    private :io=, :objects=, :first_free_object_id=, :modified_objects=, :last_object_id=
 
     sig {params(input: T.any(File, Tempfile, StringIO)).void}
     def initialize(input)
       self.io = input
       self.objects = PDF::Reader::ObjectHash.new(input)
-      self.last_object_id = objects.size
+
+      self.last_object_id = objects.trailer[:Size] - 1
+      self.first_free_object_id = find_first_free_object_id
+
       self.modified_objects = []
 
       fetch_or_create_interactive_form!
@@ -87,6 +93,16 @@ def interactive_form
       T.must(modified_objects.first).fetch(:value)
     end
 
+    sig{returns(Integer)}
+    def find_first_free_object_id
+      return 0 if last_object_id == objects.size
+
+      xref = objects.send(:xref).instance_variable_get(:@xref)
+      missing_ids = (1..last_object_id).to_a - xref.keys
+
+      T.must(missing_ids.min)
+    end
+
     sig {void}
     def fetch_or_create_interactive_form!
       root_ref = objects.trailer[:Root]
diff --git a/lib/rubrik/document/increment.rb b/lib/rubrik/document/increment.rb
index 831e865..10b9819 100644
--- a/lib/rubrik/document/increment.rb
+++ b/lib/rubrik/document/increment.rb
@@ -46,11 +46,11 @@ def call(document, io:)
           io << "#{starting_id} #{length}\n"
 
           if starting_id.zero?
-            io << "0000000000 65535 f\n"
+            io << "#{format("%010d", document.first_free_object_id)} 65535 f \n"
             subsection.shift
           end
 
-          subsection.each { |entry| io << "#{format("%010d", entry[:offset])} 00000 n\n" }
+          subsection.each { |entry| io << "#{format("%010d", entry[:offset])} 00000 n \n" }
         end
 
         io << "trailer\n"
diff --git a/test/rubrik/document_test.rb b/test/rubrik/document_test.rb
index f46d377..032886d 100644
--- a/test/rubrik/document_test.rb
+++ b/test/rubrik/document_test.rb
@@ -15,7 +15,7 @@ def test_initialize_document_without_interactive_form
       # Assert
       assert_equal(input, document.send(:io))
       # FACT: the interactive form was created
-      assert_equal(5, document.last_object_id)
+      assert_equal(6, document.last_object_id)
       assert_kind_of(PDF::Reader::ObjectHash, document.objects)
 
       acro_form = document.modified_objects.find { |obj| obj.dig(:value, :Type) == :Catalog }
diff --git a/test/support/with_interactive_form.expected.pdf b/test/support/with_interactive_form.expected.pdf
index 12db41a..d8cfbdf 100644
Binary files a/test/support/with_interactive_form.expected.pdf and b/test/support/with_interactive_form.expected.pdf differ
diff --git a/test/support/with_interactive_form.pdf b/test/support/with_interactive_form.pdf
index 97a0b15..91f4e34 100644
Binary files a/test/support/with_interactive_form.pdf and b/test/support/with_interactive_form.pdf differ
diff --git a/test/support/without_interactive_form.expected.pdf b/test/support/without_interactive_form.expected.pdf
index 22b5732..2b1a42b 100644
Binary files a/test/support/without_interactive_form.expected.pdf and b/test/support/without_interactive_form.expected.pdf differ
diff --git a/test/support/without_interactive_form.pdf b/test/support/without_interactive_form.pdf
index 29e95d6..544bfe2 100644
Binary files a/test/support/without_interactive_form.pdf and b/test/support/without_interactive_form.pdf differ