The latest version of CUPS-PDF does not work in Leopard (10.5); when you go to add the printer, the CUPS-PDF printer-type option never appears. Here is how you can make it work.
The problem is that the current version compiles by default as a 64-bit executable, and it doesn't seem to play nicely with Leopard. After installing the Mac-ready version of CUPS-PDF, if CUPS-PDF doesn't appear as an available type of printer when you click the '+' in the Print/Fax System Preference panel, look in /Library/Logs/CrashReporter for relatively new files with names that start with cups-pdf (you can do this using Console, in Applications » Utilities).
Open one of these files, if you find them. If you see anything that says x_64 in there, you are probably having a bit width conflict. If so, you will need to replace the actual cups-pdf executable at /usr/libexec/cups/backend/cups-pdf with one that you compile yourself using the 32-bit flag for gcc.
Download the cups-pdf source, and compile it according to the included readme -- but add one more flag to gcc: -m32. Note that you will need a version of XCode installed that is correct for your system. For 10.5.x, that is probably XCode 3.1.3, which is no longer available at the Apple Developer Connection -- you'll have to use the version on your original 10.5 installation disc.
You'll get a warning, but the resulting cups-pdf should be 32-bit. When you replace the one that the installer put in, at /usr/libexec/cups/backend/cups-pdf, you should be fine. You may have to restart cups itself with sudo killall -HUP cupsd.
[crarko adds: I haven't tested this one.]

