So, you're someplace away from your iTunes account and want to add a PDF to your iPhone's or iPad's iBooks collection. How can you do this?
Well, the first way is you can email it to yourself. The problem with this is that your email account (either sender or receiver) may have size limitations that prevent sending large PDFs. If the PDF is too big then here's another idea...
You can use some online storage service, copy the PDF there, open it is Safari, then choose the "Open in iBooks" option. This will both open it in iBooks and copy it into your library.
I poked around on the net a bit and found that Microsoft's Windows Live SkyDrive works pretty well. You get 25GB free, although there is a 50MB per file max upload size.