If you don't have a license for Adobe Acrobat you can review the internal structure of a PDF with the free utility PDF Vole, available for download at http://www.softsea.com/review/PDF-Vole.html . After extracting the contents of the zip file, click on the Java file named, 'pdfvole_20090208.jar' to run the program. You can open one file at a time with PDF Vole.
Unfortunately PDF Vole does not give you the ability to search through the contents of the internal structure.
If you're looking for highlights or redactions try checking the objects towards the bottom of the Object Reference Table menu. Look for one with a type listed as '/Annot'.