I see this question pop up a lot for classic literature like Camus. Legally, Project Gutenberg is probably your safest bet, but the catch is copyright status. 'The Outsider' (or 'L'Étranger', often translated as 'The Stranger') was published in 1942. In many places, including the US, it's still under copyright, so you won't find a legal free PDF from a standard source. What you will find legally are older, possibly expired translations, but the more common English versions aren't there.
Your absolute best shot is seeing if your local library has a digital lending service like OverDrive or Libby. You'll need a library card, but it's a totally legal way to get a digital copy for free, even if it's just a loan period. They might have the PDF or, more likely, an ePub. Some university websites also host public domain works, but again, Camus is likely too recent. It's a bit of a hunt, but that's the landscape for 20th-century classics.
Honestly, I tried finding it myself last year and ended up just checking out a physical copy. The digital free legal options for something this specific are surprisingly thin unless you go the library route.