I've hunted through library catalogs and sketchy forums for stuff like this, so here's what I can tell you in plain terms. The phrase 'enlightenment' can mean very different things: it might be a specific book titled 'Enlightenment' (or 'Enlightenment Now' or other similarly named titles), or it might mean teachings about spiritual enlightenment in general. If you mean a modern, copyrighted book with that exact title, the honest truth is that a legitimate free PDF is rarely available unless the author or publisher has explicitly released it for free. Big modern titles are usually behind paywalls, sold in stores, or available through library services.
If you're after classic or public-domain texts about awakening — translations of older scriptures, essays from the Enlightenment era, or classic philosophical works — you absolutely can find free PDFs from reputable sources. Project Gutenberg, Internet Archive/Open Library, SuttaCentral, and university repositories host legitimately free editions or translations. Authors sometimes post chapters or even whole books on their own websites under Creative Commons licenses, so always check the author's site. For academic takes on the 'Enlightenment' era, Google Scholar and institutional repositories often have free PDFs of papers.
Now, a practical tip: avoid sites that promise current bestsellers as free PDFs; those are often pirated and carry malware or legal risk. If a title you want isn't free, libraries (OverDrive/Libby), used-book stores, and legitimate audiobook services often make it affordable. Personally, I prefer supporting authors when I can, but I love digging into public-domain translations and open essays — they're surprisingly rich and free of charge.