The Macedonian phalanxmen wore linothorax - cuirasses made from pressed linen (how these were produced is not entirely certain, but multiple layers of fine linen cloth were involved). These replaced the older bronze cuirasses, not only because they were cheaper to produce, but supposedly also because they protected the wearer better. Alexander the Great himself, who could afford armour at any price, wore linothorax throughout his conquest.

This armour would have been amended by a bronze or iron helmet and leather or bronze bracers and perhaps also lower leg armour, making the Macedonian phalanxmen about as well armoured as the earlier hoplites.

The Macedonian phalanxmen had a small shield strapped to their left arm (leaving both hands free to handle the sarissa). The first five ranks held their pikes level while the ranks behind them (which varied in numbers, but often 11-27) held their pikes at a 45 degree angle. This meant that the men in front had pikes overhead that gave them some protection against missiles.

One must also remember that archery during ancient times was not as developed as it would become during the early and later medieval times. Most sources agree that slingers were more dangerous missile troops than archers, but more difficult to use in a formation (since a sling requires a lot of room since you swing it around you and then overhead).

Considering that the Persian armies that faced the Macedonian phalanx made extensive use of missile, skirmisher and light troops (especially archers) and thier inability to counter the Macedonian phalanx, I'd like to say no, the Macedonian phalanx was not vurnurable to arrows.