Finding free legal ways to catch up on 'Invincible' takes a bit of digging these days, since the core series is done and the compendiums aren't really given away. The most straightforward path is through library services. A lot of libraries have partnerships with Hoopla, and that's where I've had the best luck. The full run, collected in those massive digital compendiums, is often available to borrow with just a library card. It's a zero-cost model that's totally above board. You don't own it, but you get a decent loan period to plow through hundreds of pages. The image quality is crisp on a tablet, which matters for those big splash pages and fight scenes.
Some folks might point you to Comixology's free trials or Prime Reading as a backdoor, but those offers rotate so much you can't rely on them having 'Invincible' at any given moment. I checked last month, and it wasn't in the Prime Reading catalog. The official Image Comics website and app sometimes have first-issue samplers or free #1 promotions for big series, which is worth a quick look just to get a taste. But for the full saga, Hoopla's consistently been the anchor. It feels like the modern equivalent of borrowing trades from a friend, just digitized and through a public institution. The only hitch is waiting if all copies are checked out, but I've rarely had that problem with the digital versions.