My devices need some time until they start backing up. There are times when my mini needs up to 20 minutes until the process starts. Did you check your Wi-Fi connection, as suggested by col.bris?
I agree with this. It's not a switch like process, it's on its own time during a full charge.
ICloud pretty much backs up everything automatically, unlike any other tablet. I'm sorry you have been unable to get yours to work. My wife had a little struggle the first time with her iPhone but once it did all is well every time now. It it's only PDFs your concerned with backing up you could try something like "Documents" and have it auto sync with Dropbox.