PDF Vole
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.
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Unfortunately PDF Vole does not give you the ability to search through the contents of the internal structure.
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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'.
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