i have synced 5 pdfs to ibooks, they are workshop manuals for my cars. the files are 320mb, 390mb, 485mb, 625mb, and 650mb
so yes they are huge compared to the largest pdf i have which is 56mb. however, everytime i sync them .. a certain one would be able to open and not the other.
for example yesterday i could open 2 volumes that now have this cover as seen in the pic. currently only one volume would open up on the ipad, but yesterday i had 3 (out of 5) of them working. i re-synced and this is what happened

so, how do i 'load' them? just keep the ipad/ibooks open? what to do please

 

You need to import them into iTunes so they can sync. Drag the PDFs to the "Books" library. They'll be convered into epub, which will take a while for large files. Then connect your iPad and make sure you're syncing the books you want to it.