Looks like you're using an embedded browser inside an iOS app. (Maybe you followed a link in Twitter or Facebook?)
You should also be able to use the pdf file.
You may already have an app which reads ebooks. Download the pdf file and see if you're offered an option for opening the file. If so, you're done! If not...
You probably already have an app which reads PDFs. Download the pdf file and open it.
Dropbox is a good way to share your ebooks between desktop, tablet and smartphone. If you see a dropbox button above, click it to load your books into your dropbox folder. Then load the file into your reader application on your device.