diff --git a/src/calibre/ebooks/conversion/plugins/mobi_output.py b/src/calibre/ebooks/conversion/plugins/mobi_output.py
index cfe03aa79e..e8aeff0ec4 100644
--- a/src/calibre/ebooks/conversion/plugins/mobi_output.py
+++ b/src/calibre/ebooks/conversion/plugins/mobi_output.py
@@ -63,7 +63,9 @@ class MOBIOutput(OutputFormatPlugin):
                    ' This option has no effect on the conversion. It is used'
                    ' only when sending MOBI files to a device. If the file'
                    ' being sent has the specified tag, it will be marked as'
-                   ' a personal document when sent to the Kindle.')
+                   ' a personal document when sent to the Kindle. Use the value *'
+                   ' to match all books.'
+            )
         ),
         OptionRecommendation(name='mobi_ignore_margins',
             recommended_value=False,
diff --git a/src/calibre/ebooks/metadata/mobi.py b/src/calibre/ebooks/metadata/mobi.py
index 427096bfe5..a8f447c7fa 100644
--- a/src/calibre/ebooks/metadata/mobi.py
+++ b/src/calibre/ebooks/metadata/mobi.py
@@ -374,7 +374,7 @@ class MetadataUpdater:
             subjects = '; '.join(mi.tags)
             update_exth_record((105, normalize(subjects).encode(self.codec, 'replace')))
 
-            if kindle_pdoc and kindle_pdoc in mi.tags:
+            if kindle_pdoc and (kindle_pdoc == '*' or kindle_pdoc in mi.tags):
                 added_501 = True
                 update_exth_record((501, b'PDOC'))