spring preload
 

Can adjusting the spring preload on the rear shock absorber make the seat higher? Or will it just make it stiffer?

My weight is 260lbs / 118KG, should I increase the spring preload from stock?

Increasing means it gets stiffer right, for heavier riders?The manual doesnt say what adjusting will do..and no recommendations of weight and adjustment.

If you increase the rear shock spring preload it will make the seat height higher, & prevent bottoming-out if 2 up or loaded, but won't make the susp any stiffer. If it's bottoming out then it's best to adjust it.
Bob

I tightened my rear spring up totally, I think it got a lot stiffer, and thats why its not bottoming out (imo)

Im arund 100kg with gear, and couldnøt imagine to loosen it one bit, but okay, I like to jump and do stuff like that, if you're just cruising on pavement its probably fine with stock setting.

But its true, if its not bottoming out, then why tighten it :)

You would want to adjust the bike so that the bike remains level with the rider, pillion, panniers on it. If the bike is low down on the rear it upsets the rake and steering geometry.