I opened a more specific thread here with the CORS details:
Short version: Electron 39.8.10 includes a custom-protocol CORS enforcement change. Obsidian’s PDF viewer loads the viewer from app://obsidian.md, but the PDF itself from another app://... origin. PDF.js then hits a cross-origin XHR/fetch block before the PDF is loaded.
That explains why downgrading only electron39 to 39.8.9 fixes it, while newer libdrm alone does not reproduce the issue.