Do you have Firefox set to use the Adobe plugin instead of the built-in viewer? You can confirm your setting using the steps in this article: [[Use the Adobe Reader plugin to view or download PDF files]]. (Note: I opened the PDF successfully in the built-in viewer on Windows, if that's helpful.) Also, there might be an update available for your Adobe plugin: * If you use a paid version of Acrobat, use its Help menu to Check for Updates to get the last available security fixes for the Acrobat X series. * If you use the free Adobe Reader, the X series is no longer receiving security updates. You can upgrade to the XI series, which has a very similar interface to the X series, or the DC series, which looks very different. Oddly, when I go to https://get.adobe.com/reader/otherversions/ it shows MacOS Intel 10.11, but not 10.12 ?? In order to get the installer for the XI series you may need to choose 10.11 as your OS. After installing Adobe Reader 11.0.10, go to its Help menu and click Check for Updates to get the latest available security fixes. Or you can use this page: http://www.adobe.com/devnet-docs/acrobatetk/tools/ReleaseNotes/11/11.0.17.html