You are holding it wrong.
You must be clicking on the wrong link. Also disable ALL book/ebook viewers/plugins in your browser. Make sure default for all types is download.
Though most books on archive.org (the wayback machine) are only scans, so PDF is best quality. The mobi, epub and text are OCR, usually without any proofing. So Gutenberg, here and other sites that PROOF the ebooks are best.
Google is the worst copyright violator in the world. They often show page by page images with overlay OCR text. Far more than is reasonable. Some Archive.org content is from Google Books.