Unable to copy PDFs to Books in Safari

Whenever I downloaded a book in PDF format using Safari, I used to get the option to 'copy to Books', which would save it to my Books Library. Now the only option is 'Open in Books', which doesn't save it to the Library. With Epub files I can save to the Library as usual, and I can also save to other apps. Could anyone please explain why? I'm currently having to save to the Library via other browsers or apps.

Thanks.

Mark