These are the front brake pads of my bike. Lately I've seen a decrease in stopping force and some shudder when braking at low and very low speeds. I took them apart and they look like this:
Do the pads look contaminated?
I agree with @Daniel R Hicks. Those pads aren't terrible, but they're obviously not new either. New pads are cheap, so go ahead and throw a new pair on and see if it helps.
It's also possible that the pads are worn enough that even when you pull the lever in all the way they fail to make good contact with the rotor. Are they mechanical brakes? That problem is especially common with mechanical brakes.
Lastly, is it possible that you got some sort of oil/fluid on your rotors? That can severely lessen braking power, and it can happen with an invisibly thin film of oil. If you think that is the case, clean the rotors with a non-detergent solvent like goo-gone.
Also, can you describe what you mean by "shuddering"?
Brake pad contamination is usually invisible. The pad compound is porous and if oil got in your caliper / on your rotor in a big way then it has soaked into your pads.
The best way to know if your pads are contaminated is to know if you rode through oil or had some sort of suspension or brake fluid leak. A visual check around your brake caliper and rotor can reveal this. If you feel or see oily film, contamination is likely.
If you didn't have any leakage incident or ride through something nasty, it could be another problem. Your mention of shuddering makes me wonder if something is loose or worn. Still, replacing the pads is a cheap and easy check.
I had a similar problem in the past, and I also investigated the possibility of brake pads contamination. But in the end the problem was that the rotor was not tight enough. If your bike is new, maybe you need to check the screws of the rotor.
Another possibility is that with sintered and semi-metallic pads if you use the brakes for long time continuously, it heats the pads and hardens their surfaces so you are likely to notice a reduced braking power.