I don't know if there is a way to get the PPC Adobe ebook Reader activated, but one possible work around is stripping the DRM (encryption). See
Adobe ADEPT DRM for PDF circumvented. This is generally thought to be illegal in the US and some other places, but it is hard to see who is hurt by this on ebooks you bought for personal use. There is no exception to the anti-circumvention laws for "software that no longer works or is impossibly hard to use", but there should be. Once the PDF is DRM-free, the PPC Adobe ebook Reader should be able to read it.