The atmospheric pressure decreases as the altitude increases.
Atmospheric pressure reduces with altitude for two reasons, both related to gravity.
The gravitational attraction(*) between the earth and air molecules is greater for those molecules nearer to earth than those further away - they have more weight - dragging them closer together and increasing the pressure (force per unit area) between them.
Molecules further away from the earth have less weight (because gravitational attraction is less) but they are also 'standing' on the molecules below them, causing compression. Those lower down have to support more molecules above them and are further compressed (pressurised) in the process.
At sea level there is a pressure equivalent to 10 metres of water pressing down on all of us all the time. This is because of the weight of the air above us in the atmosphere. When you travel up a mountain, there is less air above you in the atmosphere.
The important effect of this decrease in pressure is this: in a given volume of air, there are fewer molecules present. This is really just another way of saying that the pressure is lower. (This is called Boyle's law.) The percentage of those molecules that are oxygen is exactly the same: 21%. The problem is that there are fewer molecules of everything present, including oxygen.
Calculate atmospheric pressure using Altitude.org's altitude air pressure calculator.