It DOES download and install after all (it finally did for me) if you make sure to do the following. 1) First, before downloading the file go to Safari preferences, General tab, and UNCHECK "open 'safe' files after downloading"; after downloading, find the compressed file and double-click to unstuff it and mount the disk image "manually," as it were. (This resolved the "download decoding failed" message I was getting.) 2) Second, BEFORE unstuffing and installing make sure you have first trashed all the bits and pieces of earlier versions (or botched attempts to install this one) on your hard drive. I got this advice from HP online support (they'll give you a list of file names and places to look for them to help with the trashing). It worked the first time, on two different G3 iMacs running 10.3.3 and Safari 1.2.1. This after some long episodes of frustration. I only don't understand why HP doesn't post this information on their driver download page!
Can't yet comment on how well the various functions work. I'm curious whether this download and intall solution works for others as well.