Sometimes the digital PDF is corrupted. Japanese Mercari sellers occasionally list the original (Part number: PIO-MRZ09-ENG) for about ¥1,500 ($10 USD). Use a proxy service like Buyee to get the physical booklet.
Forums are dead; Discord is alive. The "JDM Car Audio Collective" has a pinned resource channel with a link to a Google Drive containing the manual. Pioneer Carrozzeria Avic-mrz09 English Manual UPD
The Internet Archive now hosts a scanned copy of the rare "Pioneer Carrozzeria Export Manual MRZ09 (Rev. 3)." Sometimes the digital PDF is corrupted
: A translated PDF for the MRZ series is hosted on GitHub by user btua . Pioneer Carrozzeria Avic-mrz09 English Manual UPD