Fakku Books for iPhone

Note: I say to move it to ibooks because the pdf viewer part of the downloader I used wasn't very fast and lagged a lot.

I haven't tried it.. but in theory you could use one of the many downloader apps to download the pdf and then from there import it in to ibooks(well at least the app I used to use allowed importing...)
OR
You could just use safari and it will download in browser and you can just pick to have it sent to ibooks