As far as I am aware, PDF files are, by design, the size of a page of paper. Its real name is Portable Document format and was created so that the piece of paper could be formatted so it appears the same (and prints the same) regardless of which computer system is viewing it.
All I can suggest, then, is to take a screen shot of the file with your iPad and then use a photo app to resize the screenshot to be smaller than the normal.
Sorry, but that's all I can think of...
Marilyn