So first off, don't take my word on it, without buying the book you can look up all the rules for free (and legally) at the Archives of Nethys (link to the Beginner's Guide).

In short, while Pathfinder 1 is indeed "D&D3.75", with some fixes to some elements that break in 3.5, it's mostly more of the same and a lot of PF1e players use(d) 3.5 material to add to their games. It was still enjoyable like 3.5 but if you already own 3.5 stuff there's no real point in changing outside of wanting to change (because of WOTC being asses or whatever). They also later brought Starfinder, which is mostly Pathfinder 1 in space, with a couple new rules (mostly Stamina as a secondary, easy-to-recuperate, second health bar). Quite enjoyable also.

Pathfinder 2e is pretty different, but also similar. As others mentions, everything is much tighter ("every +1 counts", only 3 possible types of bonus), so you will not have munchkins breaking the game and forcing an arms race on the other players and the DM; and also, the encounter calculator works! As a DM, it's much easier to prepare for. They kept the large customization core, which differentiates it the most from it's main concurrent D&D5/5.5e, so you still have at least one choice at every level up, usually two. As a player, I really enjoy how you can make the same class/ancestry (race) combination feel very different. And there's like 28 classes now. Technically even more since Starfinder 2e is also out and is mostly cross-compatible.

There are of course some weak points, and I'll gladly expand upon them and any other points it if you're interested. But I've really enjoyed my time in PF2e on both sides of the screen and consider it my TTRPG of choice currently, even as a 3.5 Grognard.