You can do this with apple's own app Newstand I believe. I think I added them by putting them on dropbox and then opening them in Newstand.
Edit: this was renamed "News" in IOS9. Or maybe it's eBooks. if/or
You can also do it using iCloud. Put the PDFs/ePub on iCloud, enable that on your Ipad for backup etc and then when you open the file on iPad it should add it to News or whatever it's called now (or iBooks). There's a few suggestions here: https://softorino.com/waltr/other/trans ... ing_iBooks
But personally I use Dropbox because strawberry float having a device manufacturer/OS-specific vendor for that stuff, it works with everything and you can read the same PDF on anything else.
And I think you can do it with iTunes, even on Windows (I vaguely remember doing that before). You have to connect the iPad via USB, then it shows up in Itunes with a "Books" section even though there isn't one in normal iTunes use for some reason (because it's for MP3s and not syncing loads of other gooseberry fool like apps, remember when you had to use ITunes to put apps on an iPhone? Yeah, I guess they realised that was strawberry floating stupid but the functionality is still there.)
- Open iTunes & plug in your iOS device.
- Then check “Manually sync files to
iPhone iPad” in your iPhone iPad’s tab.*
- Locate ePUB or PDF file that you want to transfer to your device.
- Drag the ebook from your computer to
iPhone iPad in the left sidebar of iTunes
- Go into iBooks.app and select your ebook.
* I think they mean when you connect the iPad click on the parent of that tree menu thing on the left and there's a checkbox for automatically or manually syncing all your gooseberry fool
I only have an iPad so I've definitely done this before it's just weirdly obtuse.