"Open in iBooks" from Safari causes PDF to open in my app instead.

On my iPhone 6 if I go to a web page in Safari with a PDF, and press the "Open in iBooks" button at the top of the screen then the PDF opens in my app, rather than opening in iBooks.

It works fine on my iPad.

Can anyone think of why this would be, and how to change it back to opening in iBooks.

Thanks