Amazing app
Response from developer
We've made a mistake by making the latest update available to older iOS versions. You can download an older version of PDF Cabinet by offloading the current version and downloading it again. Please go to Settings > General > iPad Storage > PDF Cabinet > Offload App. This should delete the current version from the iPad, but your content will be safe. After that you should be able to tap on the PDF Cabinet icon, and the correct version will be redownloaded from the App Store.