PSPDFKit is an SDK/library/framework that you can integrate into your own apps to display PDF documents. It's the leading cross-platform solution and used by companies such as IBM or Dropbox.
PSPDFKit is a large project that started in 2010 and 35 people are currently working on it. PDF is quite hard to get right.
Interested in trying out the demo? Start here.
You can license PSPDFKit with a yearly subscription fee. Learn more about this here.