In the underlined paragraph first it is written that the protons accumulate in the lumen and in the next sentence it is written that the PH of the lumen decreases ?

Dear student,

Proton is basically H+ ion. Thus when proton increases, it means the pH decreases.
Since pH below 7 is considered acidic. (the lower the pH the more the acidic content or more the H+)
Hope this helps.

Regards