Google Books and Amazon often provide "Look Inside" previews for rare books. You cannot download the PDF, but you can view approximately 20% of Necronomicon 2. For a desperate researcher, this is a legal way to see key plates like Spell V or Nebula Over Zurich.
These are the two most common sources. Search for "Giger Necronomicon 2" on LibGen. You will likely find a file labeled "Giger_Necronomicon_2_hr.pdf." Be warned: Download speeds are slow, and the file is often split into two parts (Part 1: Plates; Part 2: Text/Appendices). hr giger necronomicon 2 pdf
Disclaimer: This article does not condone piracy. Supporting the Giger Estate by buying used originals or authorized museum editions is always the best practice. However, for archival and research purposes, here is the landscape. Google Books and Amazon often provide "Look Inside"