If you have an iPad 1, you are operating with a dinosaur. The iPad 1 changed computing as we know it (and arguably led to large Phones), but the iPad 2 was an evolutionary jump over the iPad 1.
The iPad 1 stopped receiving updates for iOS some time ago; and most developers do not write apps that can run on the iPad 1.
It is too bad that Apple doesn’t offer a “legacy” store for these older devices–which still work. Sadly, that option does not exist. When a developer abandons the device, the old apps can no longer be purchased.
There has been one PDF music reader on the market–unrealBook–that has remained compatible with the iPad 1. Most developers gave up on that device long ago–and that means every other PDF music reader. Yesterday the developer, Aron Nelson, announced that he had reached the point that iOS 9 is now so complicated that it is nearly impossible to keep developing for the iPad 1 at the same time.
In other words, this last version of unrealBook will be the last update that works on an iPad 1.
If you know anyone with an iPad 1 that wants to use it as a PDF music reader, they need to buy this app now, or they will not be able to do so in the future.