diff --git a/domain/bulk_document_download/package_plan.py b/domain/bulk_document_download/package_plan.py
index bcd122aad..535928596 100644
--- a/domain/bulk_document_download/package_plan.py
+++ b/domain/bulk_document_download/package_plan.py
@@ -8,6 +8,7 @@ orchestrator does the S3/DB work; this module only lays out the archive.
 
 from __future__ import annotations
 
+import posixpath
 from dataclasses import dataclass
 from datetime import datetime
 from typing import Optional, Sequence
@@ -57,6 +58,14 @@ class DownloadPackagePlan:
     skipped: tuple[SkippedDocument, ...]
 
 
+def _extension_of(s3_key: str) -> str:
+    """The file extension of an S3 key (``.pdf``, ``.zip``, …), empty when the
+    key's basename has none. Used to keep the packaged file openable while
+    naming it by Document Type."""
+    _root, ext = posixpath.splitext(posixpath.basename(s3_key))
+    return ext
+
+
 def plan_download_package(
     documents: Sequence[ResolvedDocument],
 ) -> DownloadPackagePlan:
@@ -83,7 +92,7 @@ def plan_download_package(
     entries: list[PackageEntry] = [
         PackageEntry(
             zip_path=f"{document.address} ({document.landlord_property_id})/"
-            f"{document.document_type}",
+            f"{document.document_type}{_extension_of(document.s3_key)}",
             s3_bucket=document.s3_bucket,
             s3_key=document.s3_key,
         )