Well, I am a post graduate studying mathematics. Current I am working on the mathematical theory of star(not an expert thought). When I am making some assumption on the equation of state (for a star or sun of course), I simply sum up the radiation pressure, idea gas pressure and degenerate pressure together. In some sense I believe this choice is an appropriate approximate to some model. But when someone ask me why, I am kind of stuck, even thought some of the lecture note posted online supports my guess(for example, http://www.jb.man.ac.uk/~smao/starHtml/equationState.pdf). But I am still wondering, is there any standard text book introducing something like this? I know Chandrasekhar's book has already told something, but it looks like it did not really state that I can simply sum all the pressure together linearly. Thank you in advance.
You cannot just add the ideal gas pressure and degeneracy pressure together for any particular species of particle. (Well, you can, but it would be a poor approximation to the actual pressure).
The trouble is that they are not separate things. In general, the electrons in the gas are partially degenerate and always obey Fermi-Dirac statistics. Unfortunately there is no analytic solution that gives you pressure as a function of temperature and density; you either have to use Tables of pre-calculated results, or there are analytic approximations that work well in certain regimes.
Clayton's Principles of Stellar Evolution and Nucleosynthesis contains some of these tables and also some better analytical approximations that work in certain regimes of (partial) degeneracy.
What is true is that you are fine to add to this pressure, the radiation pressure due to photons and the ideal gas pressure of the ions.
EDIT: I realise that what may be troubling you is not the above, but why you can add the separate pressures together at all (eqn 122 in the notes you reference). The reason can be traced to eqn 31 in the same notes. This says that the pressure is an integral over the number density and momenta of individual particles. Note that this equation does not contain anything that identifies the particles (i.e. their mass). The integral can therefore be treated as a summation over all particles, which in turn can be separated into three integrals that cover the ions, electrons and photons separately.