Dropbox and Goodreader are your friends.
If you have a Dropbox account you can have it on your PC and iPad. Put all the manuals you want on your iPad into Dropbox on your PC, then they will also be available in Dropbox on your iPad. You can then " Open" them in the Goodreader App as "books" or " documents".
It will work with using "Cloud" app commonto both PC and IPad.