“many...many.” Even if a person lives “many years,” the years after death will be many also. the repetition of “many” makes the sentence catch the attention.
“let him rejoice in them all.” Many verses in Ecclesiastes encourage people to rejoice and have fun in life (cp. Ecc. 2:24-25; 3:4, 12-13, 22; 5:18-19; 8:15; 9:7-9; 10:19; 11:7-8). See commentary on Ecclesiastes 2:24.
“let him remember the days of darkness.” In this context, the “days of darkness” are the days when the person is dead. The verse is saying rejoice in the life you are living, but remember death and the Judgment Day that follows it, and live wisely.
“pointless.” See commentary on Ecclesiastes 1:2. Here we see the meaning of “temporary” coming more to the front, but the primary meaning still is “pointless,” especially in light of what the Sage already said in Ecclesiastes 1:11, that even the things that are still future will not be remembered by the things that will come after them.