It works!

src/documents/admin.py

@@ -0,0 +1,24 @@
+from django.conf import settings
+from django.contrib import admin
+
+from .models import Document
+
+
+class DocumentAdmin(admin.ModelAdmin):
+
+    search_fields = ("sender", "title", "content",)
+    list_display = ("created", "sender", "title", "thumbnail", "pdf")
+    list_filter = ("created", "sender")
+    save_on_top = True
+
+    def thumbnail(self, obj):
+        return '<img src="{}documents/img/{:07}.jpg" width="100" />'.format(
+            settings.MEDIA_URL, obj.pk)
+    thumbnail.allow_tags = True
+
+    def pdf(self, obj):
+        return '<a href="{}documents/pdf/{:07}.pdf">Download</a>'.format(
+            settings.MEDIA_URL, obj.pk)
+    pdf.allow_tags = True
+
+admin.site.register(Document, DocumentAdmin)