Linen trousers are the one thing that makes a hot office bearable, and that's because nothing else lets air near your legs while still looking like you made an effort. I spent a few summers sweating through polyester tailoring before I worked that out, and I'm still annoyed it took that long.
They come into their own from late spring through to the end of a warm September, and they're particularly good for the kind of day that starts cool and ends up close and sticky by three. A wedding in a marquee, a work trip where the aircon's broken again, a Saturday that involves a lot of walking and no real plan. They're less convincing in deep winter and completely pointless in driving rain, so don't buy your only pair in navy and expect them to cover you for the whole year. If you're starting from nothing, go for a straight or slightly wide leg in a mid weight linen, in stone or a soft olive rather than white. White shows everything, including the outline of whatever you're sitting on, and I learned that one the hard way on a plastic chair at an outdoor lunch.
What they pair with depends on how much structure you want to borrow back. A fitted shirt or a simple top tucked in gives you something that reads as put together without trying. A linen blazer over the top, if you've got one, turns the whole thing into something you could walk into a meeting in. For anything more relaxed, a plain vest and flat sandals does the job and does it quietly. The trousers do enough of the talking on their own, so you don't need the rest of the outfit shouting as well.
Fit is where linen trousers either work for you or sit in the back of the drawer by August. Go for a rise that sits properly on your waist rather than low, because linen has no interest in holding itself up and will slide if it's cut wrong. Check the back for any gape when you sit, since that's the fit problem nobody mentions until you've been sat at a desk for an hour. They crease, that's just linen, but a good weight bounces back after a few minutes of movement rather than staying folded in a map of lines. After a season of washes they soften rather than wear thin, which is more than I can say for most high street trousers at this price. Thrown in a bag for a weekend away, they come out crumpled but never actually tired, and a shake and a short hang sorts most of it out. Tomorrow morning I'd put mine on with a plain fitted top and the flat sandals I've had for three summers running, and call it done before the kettle's boiled.