Meaning: The document title has not been set in the metadata, meaning there is no title associated with the document in the backend under "document properties" which helps provide descriptive information about a document.
Fixing this should also automatically fix the error:
ViewerPreferences dictionary of the Catalog dictionary does not contain DisplayDocTitle key.
If you use Word to create your PDFs watch this video:
If you use Adobe Acrobat to create your PDFs watch this video: