These tablet computers from Amazon don't use the Google play store for app delivery. Instead Amazon operate their own marketplace app store. When I can cannot use Google play store how do I get the free Adobe PDF viewer app please? Some app vendor websites like Opera allow people to directly download .APK installation files.
Since Amazon rolled out their Fire OS 7.3 Adobe Acrobat Reader is no longer supported. The default message from Amazon says the developers of the app have been contacted and the new version of the app will be available on their App Store once the developers make it available.