It's a scam.
No matter what you download, all you get is a pdf (just change the suffix to .pdf and you can open it as normal) called "HOW TO DOWNLOAD A FILE FROM THE INTERNET?" containing loads and loads of mixed-up crap interspersed with fake links to what you are actually looking for.
They harvest book titles from various book sales websites in dozens of languages and on every conceivable subject, and pretend to have them as pdfs, but no matter what you download they are all the same - the only thing that is different between them all, is that the fake links are changed to include "your" book.
I have come across other places like this pretending to have pdfs of obscure academic books - and all you get is the "tutorial" on how to download a file from the internet....
And I wouldn't touch their "exbook reader" with a bargepole.