Loading PDFs on iBooks?
 

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.