Text compression algorithms like Zip, 7Zip, RAR, et. al. compress data by identifying repeating character strings in a segment of text and substituting pointers to the original string instead of repeating the string itself. Graphic and audio files have few or no such repeating strings and therefore are not compressed by these algorithms.
I'm guessing that's the answer. The file was one large PDF consisting of several pages of documents scanned in. I assume this means there would not be any of the character strings needed to reduce size through the repetition.
Just out of curiosity, I ran a quick test and compressed a text-only Pages file. It went from 180 MB to 147 MB. Thanks for the comprehensive and clear explanation....the penny has dropped.