Isso significa que o arquivo está criptografado.
Para citar Steven Madwin, um funcionário da Adobe:
the application puts it in the title bar as a visual clue that the user is in an encrypted file. The default is for the application (either Acrobat or Reader) to display the file name, encryption status, and the application name in the title bar. You can replace the file name with the document title, but you cannot suppress the encryption status. This was added to Acrobat 8 because too many users were viewing encrypted PDF files and didn't understand why they couldn't make changes.