How to download a PDF from Viewer.js?

I want to download a PDF file from this site.

It can only be viewed from a flipbook which have a restricted access. A few months ago, I got the PDF source (address.pdf) from the inspect element easily. But now it seems that I can’t find it anymore.

I’ve tried the inspect element network and properties method, but I still can’t find a way to download this file. Thanks for your help in advance.