what's the best way to save a pdf to 'local' in Ipad?

So far the only way it works is saving from email attachment. I'm looking for a way to save pdf's to local. I have a very large library and since I travel a lot, I don't always have wifi. Does anybody has tried or knows a way of going around it?. When opening files either from document cloud or from dropbox, I can read them for sure, but they don't give me a way to save to 'local', so I can have them available offline.