PBS damage applies to each arrow from a Manyshot and the +1 AB applies to the roll.
I'm bored, so I'll toss in my manyshot rant here.
People always complain that manyshot is a useless feat in the presence of rapid shot (and improved rapid shot). Those people need to have their heads examined. With a mallet.
The single, most power feat for a fighter type is power attack. Power attack allows you to exchange accuracy for more damage. Some people scream bloody murder when you can give up a +4 on your attack roll to get a +8 on damage.
Power attack can be used in a single attack, a full attack or an AoO (as well as with certain special maneuvers). With regards to a full attack, it is often of minimal help becuase it requires you to give up accuracy on attacks that have a low chance to hit as well as on attacks with a high chance to hit. The average reduction in accuracy per full attack tends to be high enough that the average increase in damage per hit does not make a significant change on the combat damage or, in many cases, reduces the average expected damage during a full attack. Thus, power attack tends to be most useful in a single attack, on AoOs and on some special maneuvers which are very similar to single attacks (sunder, etc ...) There are a few full attack situations where power attack is *significantly* useful, but not too many.
Manyshot has much the same use as power attack. You give up accuracy in a standard single attack to increase the damage. Except, instead of giving up 4 points of accuracy for 4 points of damage, you give up 4 points of accuracy for 4.5 points of damage + non-precision accuracy based bonuses + strength bonuses + magic bonuses - often about 10 points of damage. A far better exchange than power attack.
Many people say that power attack is less useful because it delivers the damage in many small pockets instead of in a big pocket. That makes manyshote less useful against DR. That is true. Except that archers should be able to avoid DR most of the time. Unlike melee types, an archer can easily carry a DR piercing set of weapons for each DR type (well, perhaps not bludgeoning). Where it would be impractical for a melee fighter to carry a magical & holy & lawful cold iron great axe, a silver chaotic great axe, an adamntine great axe, etc ... an archer can stock a small supply of each of these arrows in his quiver (preferably a magic quiver to hold a lot of arrows, though extras can be stored in a bag of holding or other storage device and be retrieved between battles). If your archer is running into DR trouble too much, you're either playing in an unusual campaign with low resources for PCs (or very odd DRs are present) or you're not planning well enough. Yes, when you do run up against DR, manyshot is less useful than PA. THe lesson - don't run into DR that you can't avoid.
Others point out that you use standard actions less often as an archer than as a melee fighter. Melee fighters often have to move to meet their foe, while an archer can stand behind the front line and fire. This is true, but not to the extent that most people think (at least not in my experience). Archers can often attack in a surprise round when other PCs are moving towards their foes. Archers often have to move to negate cover issues, especially when someone has total cover. Archers often have to move to avoid foes that bypass the front lines to go for the archers and spellcasters in the rear.
In the end, manyshot is a very useful feat. If you don't think so, try it out.