PDF will not save in Books

I am able to download and open a pdf in Books on my iPad (the default program that comes with iPad), but once I’m done reading it, it disappears. I can’t find it in my Books library or downloads, or anywhere else within Books. I have not had this issue with other PDFs. Usually when I download a PDF to Books (which I’ve done many times), it is saved automatically to the downloads area and I have the option to move it to another folder. But, with this particular PDF, I am only able to read it once, and then it disappears, nowhere to be found within Books, until I redownload it again online. I’m pretty sure it’s not a space issue, since I recently reset my iPad and have almost nothing installed on it. Anyone know whats going on here?

The PDF in question is the “downloadable rules” pdf available on this page:

https://ionsmg.com/products/bios-origins-second-edition

(It’s the rulebook for a boardgame. I’d post the direct link here, but it links to a Dropbox folder and so the direct link address probably looks scary.)