If you're on an iPad, you may not be able to open the eBook.
I've no idea why. I have an Android tablet and and Android phone and a computer, and I can open the file on everyone of those. It's a normal PDF file.
Normally, I download a PDF to my computer first.
Then I open it on my tablet or phone (or computer). Sometimes, I open it on my computer and asks Adobe Reader to save the file to the cloud. That makes it super easy for me to read on my Android phone or tablet.
You can also export to Kindle if you have one. I do. I'm not a big fan of PDF on my Kindle, but it's doable.