What happen to pH of distilled water if we add lemon juice to distilled water?

You need to study this section in your book until you understand it perfectly. Lemon juice contains citric acid, as any citrus fruit does. Acid means pH below 7. Water is neutral, pH at or near 7. So adding any acid to water lowers the pH, which means it increases the concentration of H+ ions.

pH=7 means 1 H+ ion per 10 million molecules.
pH=6 means 1 H+ ion per 1 million molecules.
pH=5 means 1 H+ ion per 100,000 molecules.
pH=4 means 1 H+ ion per 10,000 molecules.
pH=3 means 1 H+ ion per 1,000 molecules.
pH=2 means 1 H+ ion per 100 molecules.
pH=1 means 1 H+ ion per 10 molecules.
Get the picture?

Pure lemon juice has pH about 2. So if you mix one part lemon juice with 9 parts water you would expect the mixture to have pH about 3.