How do I delete transfered PDFs on PC but not iPad? Need space!

I have succesfully gotten a group of large PDF photography books from my PC onto my ipad, and now I would like to delete them from my PC (yes, they are backed up) to save space. I read somewhere that I should delete them through iTunes rather than in their original location, and that I would be offered a choice of deleting them only on my computer or both online and on the hard drive. I tried several times, and each time was told that deletion would also erase the files on my ipad and iphone (at the moment I am able to open them in iBooks). So, I didn't do that but I sorely need the space! Can you help? 😕

(BTW, I did post this question at the end of a long thread on transferring PDFs a few days ago, but since there was no response I thought it was probably not the right place.)

Thanks from a new ipad/iphone user who still has a lot to learn!