Why does a tablet usually last longer than a laptop with the same usage (eg read pdf)?
Because sometimes they have other batteries that last longer.
A tablet usually has very weak hardware that consume very little power. Also, the TFTs are usually smaller and require less power.
Then it depends on the OS.
Example:
A tablet consumes 5W in load.
A notebook 50W in load.
Because tablets tend to have more fuel-efficient ARM processors that use less power than the x86 processors on laptops.