This drives me nuts too. The problem is that cut marks add a little bit extra because instead of being just corners, it makes misshapen plus signs and reduces the printable area to accommodate the outward-facing pieces of that plus sign. The only way around it I've found is to reduce the size of the PDF page that's being poster-ized until it stops overflowing to more pages. I actually went through this this weekend and got so frustrated I installed alternative PDF viewers to try to get a proper tiled print. I found that Foxit Reader seems to have slightly smaller cut marks that allowed me to get my 23x29 plans onto 9 pages instead of the 15 or so that Adobe wanted.
That said I installed them into virtual machines because I am wary of free PDF viewers installing spyware and I can't say with 100% certainty that Foxit is completely clear.
To get around this normally, I just import the PDF into Inkscape and copy out bits into smaller PDFs. This works well with vector drawings, but I don't think it would work with a scanned PDF. I'd be very interested to hear if there's a better way to do this considering I spend a lot of time taping plans together and often end up with parts with non-straight edges due to my poor tile assembly.
