PDF not scaling correctly

In my experience, it is that the pdf in question was sent to you that way. Example: original document should be a 36"x24" pdf but was exported as 8.5"x11". I can't tell you how many times I've been sent this nonsense. You'll have to maually scale with the "by length" rather than ratio. Unfortunately this will most likely distort the image quality somewhat.
The only other option is to request the properly scaled pdf's.

Hope this helps,

Nathan