Although "Adobe Reader 9.1" is outdated, you can still get the latest version of the Adobe Acrobat Reader app for your tablet through official channels: How do I install Adobe Reader on a Tablet? | Community

For users looking to view PDFs on their device today, the best solution is to use the modern, officially supported or the Fire tablet's built-in PDF viewer . Official Ways to Get Adobe Reader on Kindle Fire

Adobe Reader 9.1 is a legacy software version originally released in 2009 for desktop operating systems like Windows XP and Mac OS X. While modern Kindle Fire tablets (now officially known as tablets) are based on Android, Adobe Reader 9.1 was never designed for mobile tablet architectures.