Why does pH change with temperature?
Water always contains a certain amount of $\ce{OH-}$ and $\ce{H3O+}$ ions. The pH is the negative decadic logarithm of the $\ce{H3O+}$ concentration (pH = - log(c($\ce{H3O+}$)). As chemical equilibria might change with temperature so does the concentration of $\ce{H3O+}$ and therefore the pH.

Although I never tried, I suppose this could affect measurements using pH paper. On the other hand the pH changes might very well be to small to be detectable using this method.

How this affects other fluids / equilibria strongly depends on how each equilibrium is affected by temperature. At higher temperatures equilibria are shifted in the endothermic direction and at lower temperatures they are shifted in the exothermic direction. You will need to look up the details for each equilibrium I suppose.