Books in iOS 13 (since iOS 12)

A similar topic has been brought up before, but I didn’t find the solution. The issue I’m having (both in iPhone and iPad) since iOS 12 and it continues now in iOS 13 is that sometimes when I open a pdf document from a website, the “copy to books” option is missing. Rather it says “open in books”. At first I didn’t catch the difference in wording. But the difference in how it responds is huge for me. Basically, the “copy to books” option (when it does come up) not only opens the file in books, but once you close that file there’s a copy kept there for future use. In fact, if one is using iCloud backup it even uploads that file to your cloud.

However, the “open in books” option does JUST that. It opens the pdf file in books but nothing more. Once you close the file it’s gone; you have to keep downloading it from the website you obtained it from, or at the least reload the page. My question is: why do some pages only have the “open in... “ versus the “copy to... “ option? Also, how can I make it so “copy to... “ behavior is the default? Ultimately, I want to keep a copy of the pdf file if I’ve opened it in books.

Thank you for your help.