If you have a valid, perpetual license for Acrobat XI Pro installed from a DVD or original download, the 11.0.27 patch would bring your software to its most stable (though still obsolete) state.
Users reported that pre-11.0.27 versions would crash when validating PDF/A (archival standard) documents. This patch stabilized: Adobe Acrobat XI Pro 11.0.27 Patch
Adobe Acrobat XI Pro (version 11) is an end-of-life (EOL) product. Official support, including security updates and patches, ended on October 15, 2017 Adobe Help Center Critical Status & Security No Further Updates If you have a valid, perpetual license for