The clue's in the name, I guess: the job of the adventures (and the adventurers) is to find the path. The path goes from point A to point B to point C, in order. It follows a plot, in the original sense of the word: a line plotted on a chart to show the route that something will follow. First this happens. Then this happens. Then this.
The characters move through space, but that's basically incidental. The whole adventure is plotted out in terms of time, and travel through time is (almost) always one-way. Even when they are plotted out spatially instead, they are often arranged in such a way as to heavily encourage a linear approach: long, narrow dungeons, encounter areas arranged along a road that runs from A to B to C, and so on. Freedom of movement is usually kept to a bare minimum.
|A 'plot'. You will start at Paris and end up at Limoges. You will go through Barcelona, Seville, and Porto. You will not visit Madrid along the way.|
In most OSR adventures, by contrast, what's presented is space. A bunch of things are happening in a bunch of places: which ones you go to, and in what order, and what you do when you get there, is up to you. Instead of saying 'first this happens, then this happens', they say 'in this place this is happening, and in this place this is happening'. The primary movement of the characters is through space, which they are permitted to navigate freely, in three dimensions, rather than simply being marched down the one-way hallway of linear time.
What keeps striking me about the better-written Pathfinder adventures is how easy it would be to blow them open. Arrange them across space instead of time: turn scenes 1-10 into locations 1-10, and let the PCs wander between them at will. (I did a version of this for the Kingmaker adventure path here.) Pathfinder usually can't do this because its system is so restrictive that any encounter more than a couple of levels lower or higher than the PCs is going to be a cakewalk or a TPK, meaning that everyone always has to meet everything in exactly the right order. But OSR-style games don't usually have that problem, and in any case, most OSR players will understand that not everything is there to be stabbed in the face.
Take the first two chapters of Rise of the Runelords. As written, they're completely linear - first the PCs fight the goblins in the town, then they fight the goblins in the glassworks, then they explore the dungeon under the town, then they go to the goblin lair, then they investigate some murders, then they fight the ghouls in the farmlands, then they explore the haunted house, then they take on the cultists, then they assault the clocktower. But given a less restrictive system, all that content could be made available simultaneously. The PCs ask around in town, and they hear about weird murders and goblin attacks and old smuggler's tunnels and the big haunted house outside of town - and then what they do next is genuinely up to them. This also means that the clues to what's really going on can be uncovered in any order, leaving it up to the PCs to join the dots and work out what it all means, rather than having it all artificially spoon-fed to them in the 'right' order by the GM. And it means that the intra-NPC interactions really open out into a genuine web of relationships, because rather than only encountering each villain after the last one is safely dead, the PCs are free to ally with them, manipulate them, turn them against each other, and generally behave like the scheming little monsters they inevitably seem to be.
So if you've ever read a module which you quite liked in some ways, but were put off by its railroaded linearity, take a moment to think: can you un-railroad it? Sometimes the answer will be 'no': sometimes scene C requires scene B to have already happened, which requires scene A to have already happened, in which case nothing but a complete rewrite will save the adventure from being a total railroad. But sometimes modules pretend to be a lot more linear than they really have to be, and with only a small number of changes - essentially just a rearrangement of the same material across three-dimensional space rather than linear time - they can be turned into something much more in tune with an OSR sensibility...