iOS includes a Share sheet action for saving PDFs to iBooks. This is great for, say, turning a webpage or other document into a PDF form with one notable complaint—not everyone wants to read PDFs in the Books app.
As it turns out, you can export a PDF to any app on your device with 3D Touch right from iOS's Print Preview screen. This is an extremely convenient feature, but it's unfortunate that it requires 3D Touch so owners of previous-generation and older iPhones cannot take advantage of it.