Try this way

src/documents/consumer.py

@@ -501,7 +501,7 @@ class ConsumerPlugin(
                         pk=self.input_doc.root_document_id,
                     )
                     self.log.debug("Saving record for updated version to database")
-                    original_document.pk = None
+                    setattr(original_document, "pk", None)
                     original_document.root_document = root_doc
                     file_for_checksum = (
                         self.unmodified_original

src/documents/models.py

@@ -161,8 +161,6 @@ class Document(SoftDeleteModel, ModelWithOwner):
         from django.db.models.query import QuerySet
 
         versions: "QuerySet[Document]"
-        pk: int | None
-        id: int | None
 
     correspondent = models.ForeignKey(
         Correspondent,