And finally, ... there is at least one further PDF reader for Mac OS 9

"MacGhostview" in version 3.2 from 2005. Shareware, and the developer himselve only offers OS X versions anymore, but it´s downloadable here:

http://www.mth.kcl.ac.uk/~anderl/mac/macghostview32.sit.bin

workes pretty well at my G4