If you want to read 'Treasure Island' for free, the cleanest and fastest route is Project Gutenberg. I often go there when I want a no-frills, reliable text — they host the public-domain edition, which you can download as plain text, EPUB, or Kindle format. The version is faithful to Robert Louis Stevenson’s original, which makes it perfect if you want the classic language and pacing. Another excellent archive is Internet Archive, where you can find scanned copies of older illustrated editions; flipping through an N.C. Wyeth-illustrated scan on my tablet felt like discovering a dusty old map all over again.
If you prefer listening, LibriVox has volunteer-read public-domain audiobooks — some narrators are theatrical, others are calm and steady, so you can pick what suits your mood. For those with a library card, Libby/OverDrive and Hoopla are lifesavers: many libraries offer free digital copies and audiobooks of 'Treasure Island' and other Stevenson works through those apps. Open Library also lends scanned copies if you create a free account, and ManyBooks and Google Books sometimes host formatted editions as well.
A little tip from my experience: check edition notes and introductions when you download, because some online versions include helpful annotations or old spelling that changes how you read sentences. Also hunt for illustrated editions if you want that extra visual atmosphere. I still get a thrill when Long John Silver first appears — reading it on a rainy afternoon felt like being on deck again.