I've run into this bug. On my 10.3.8 G3 iBook installing Acrobat 7 results in blank pages instead of a PDF display when I open a PDF file in Safari. Here's a fix I'll try:
"For Safari, Acrobat 7 attempts to 'take over' the chore of displaying PDF's from the default Preview.app program. For some reason, this does not work, and Safari is left unable to display PDF's at all.
I would bet good money that Tom will find the following file on his system: /Library/Internet Plug-Ins/AdobePDFViewer.plugin
If he deletes that file, Safari should then be able to display PDF's again (via Preview.app).