Are Almanac page paths predetermined?

Yes. The path of an almanac page is predetermined, so you can learn where it will go, beat it there and then just wait.

They tend to follow obvious free running paths anyway, so they're generally not too hard to catch unless they go over things like rooftops and the like where you don't have a path to follow.

I've found that an easy way to get them is to get on top of an adjacent building, rather than climbing the building that they're on the top of, as they'll start to move while you're still climbing. If you're already on top of the structure, it'll start to move and you'll already be able to free run, and can catch them no problemo.