How to retrieve a PDF file from an iOS device?

Given two Macs running Yosemite and one iPad running iOS 8, how do you retrieve a PDF file put on the iPad from Mac 1, using Mac 2 ? Let's say that I don't currently have access to the Mac used to put the file in the first place. I can see the file in the Books section of the device in iTunes, but that's all I can do right now.