As you suspect, PDF is "pretty solid" already.
In fact PDFs main difference with its precursor, PostScript, is its lightweight data structure (less wasted space) and the internal use of several compression algorithms including RLE & LZW... so GZipping most PDF's doesn't reduce there size very much at all.
BUT... PFDs are by nature "static content", so if you combine compression with response caching (especially of very large documents) you could be onto a winner... On the downside, I suspect some browsers won't recognise compressed-PDF, they'll see Content-Encoding "application/pdf" and ignore the additional "gzip". Life's a gamble ;-)
More here: http://www.verypdf.com/pdfinfoeditor/compression.htm