An "unknown error" uploading to ChatGPT often means the file is problematic (e.g., bad format, corrupt, secured, too big), your internet is unstable, or there's a temporary browser/server glitch.
Quick Fixes:
File: "Print" the PDF to a new PDF, zip it, or use OCR if it's a scanned document.
Browser: Clear cache/cookies, try incognito, or use a different browser.
Connection: Check your internet or try uploading later.
