The IEEE PDF requirements do not allow PDF bookmarks. You need to use Acrobat or other tools to remove the PDF bookmarks or instruct your PDF generation software not to include those bookmarks. This warning only applies to final manuscripts and can be ignored for review manuscripts. When converting Word files to PDF using Adobe Acrobat, you can disable bookmark generation by removing the check mark from AdobePDF > Change Conversion Settings > Bookmarks > Convert Word Headings to Bookmarks.