The present invention relates to a process for manufacturing paper or board, wherein paper or board pulp is diluted with acidic water and wherein the pH value of the pulp is raised with an alkali simultaneously with increasing the solids content of the pulp by filtration, compression and evaporation on the wire, press and drying sections, and, at the same time, filler is precipitated from the acidic water into the paper or board structure. The invention also relates to moistening of paper or board, with a dry matter content of over 40%, in acidic water, after which the pH value is raised with an alkali, and the paper or board is dried, or it is dried after the moistening without raising the pH value.