The PCT is a very long trail and the quality and quantity of water can vary considerably from one section to another.

The water sources in SoCal are notoriously scanty and can easily become polluted by cattle or other animal use. Thru hikers are often confronted with stagnant pools or troughs with scums of algae, insects, or worse. I don't think a Steripen would be adequate for such situations.

On the other hand, the Sierras often have abundant clear fresh running water that has melted from snowfields within sight of where you draw your water. For such water sources you can usually drink with no filtration or purification and be perfectly safe.

Oregon and Washington, where I hike, are not as bad for water as the worst places in California, but because of the volcanic nature of the Cascades the water sources are very often lakes, not streams. Lake water is pretty good, but I use a Katadyn Hiker filter unless I am lucky enough to draw water from a pure, burbling spring that emerges within a few feet of me.

The Steripen ought to work OK for most lake water in the OR and WA Cascades, I should guess, but there is that problem of relying on a gadget with batteries that can go dead on you, or just go haywire on you. I tend to avoid such "solutions" and rely more on simpler ways of doing things.

Whatever you choose, I wish you luck and happy hiking. The PCT is a great trail to hike!