Disappearing bookmarks in Acrobat XI trial version

Hi there, I recently converted a Word 2013 document to a PDF using Acrobat XI trial version. However, when  I reduced the PDF size and secured it, the bookmarks disappeared. I'm interested in buying the full version but I need to know first why this is happening and what I need to do to fix it. Can anyone throw any light on the problem? Is it a problem with the trial version only?