How can I do a red line compare of two different PDFs?

Hi,

Sorry for the problem you're facing. As of now, Compare functionality doesn't have any such feature to highlight the changes in two PDFs. If you feel that this feature should be added in our coming releases, please make a Feature Request at: Adobe Acrobat - Feature Request/Bug Report Form

Also, we have significantly improved Compare functionality in our new release of Adobe Acrobat DC. Please update to the latest version to give it a try. The new Compare Files Tool offers a Side-by-Side view which provides a better experience in identifying differences and a modern UI.

Latest version for Adobe Acrobat DC build number: 2015.020.20039 (Continuous).

For more info about the new Compare tool, visit: https://helpx.adobe.com/acrobat/using/compare-documents.html as well.

If you are still on Acrobat X or XI, you can check out our trial version at: https://acrobat.adobe.com/in/en/free-trial-download.html​

Thanks,

Adobe Acrobat DC Team.