PDFs are something of a necessary evil. Sure, they’re a handy way to preserve any document in the style you intended, and can be read on virtually any device…but if your device doesn’t have a good built-in PDF reader, finding one can be a pain.
It’s no different on an iPhone or iPad. The default method to read PDF files on these devices is using iBooks, which was included as a preloaded app beginning with iOS 8. Sure, you can view PDFs in Safari, but you won’t have very many additional options. And while iBooks is sufficient and has some nice features, there are other apps that make reading PDFs downright luxurious, and these can all be downloaded from the App Store.