You're probably using Cloudflare DNS, it's a known issue that's been discussed for years. The archive owner deliberately gives bad results to Cloudflare DNS. It's why I prefer not to use it.
The content seems taken from other websites and mixed in a nonsensical way. It comes up frequently in my search results.
www.xspdf.com has completely unrelated content and seems a separate business.
Simple website that uses the ffmpeg's removesilence filter to remove silent windows (>= 1s) from audio files.
Everything is done locally in the browser using ffmpeg.wasm.