The consensus seems to be: The PDF is a tease. Because Chuukyuu e Ikou is designed for classroom pair-work (listening to a partner, reacting to a prompt), doing it alone with a grainy scan is like learning to swim by reading a manual in a sandbox.
Eventually, I bought a used copy from a seller in Osaka. It cost me $45 with shipping. The pages were slightly yellowed, and the previous owner had already filled in the answers in pencil.
"The scans were crooked." "Page 47 was missing." "The audio files were labeled Track 1, Track 45, Track 2."
The people who succeed with this textbook are the ones who bought the physical copy. They write in the margins. They fold the pages. They suffer through the listening exercises with the actual CD. The physicality of the book forces a commitment that the PDF cannot replicate. I don't have the PDF. I looked for a long time. I dug through Google Drive links that were deleted hours after they were posted. I clicked through Mega.nz links that required decryption keys. I found nothing.
Learners search for Chuukyuu e Ikou specifically because they know it is the only structured bridge out of that desert. They aren't looking to steal from a corporation; they are looking to survive a plateau. They want a scaffold. And when they can't find the PDF, many of them quit.