On threat range, things are written such that you really only get one buff at a time and they don't stack with each other.
As for damage look at d20pfsrd.com at the feat tree under the rapid shot\fire line of feats and the feats that sprout off it. You can buff the bow's enhancement bonus, give it weapon qualities such as bane and seeking.
Stuff like that.
Ask your DM about buying magic arrows in sets smaller than 50. When the halfling ranger in my Council of Thieves campaign went looking for ammo, I would let him buy in lots less than 50 - in fact - I more or less made him do so because I would roll 1d20 or 2d20 to see how many of a particular ammo was available in the city when he went looking for it. I just prorated the cost with each piece of ammo being 1/50th the cost of the whole set.
Bane ammo, particularly stacked with your own favored enemies, are particularly useful as are special materials like silver and adamantine because they help you get through DR. Holy arrows are also really nice.
Manyshot also turned out to be pretty useful for the halfling ranger. He used a staff sling and his strength was actually pretty good, so he was doing some highly respectable damage with it.
Erm... a bow can't be keen...
There is no good reason to disallow ways to enhance any weapon's crit threat range in ways exactly like keen. And it sounds like the character is already using it so that implies the DM has approved it.
Most of an archer's damage increases come from feats - precise shot/improved precise shot to minimize attack penalties, rapid shot/manyshot to fire more arrows, deadly aim to do more damage with those arrows.
If you seem to be facing the same type of enemy repeatedly, you could buy some arrows with bane on them. The other option is flaming, frost, or shock arrows for extra damage.
As an aside, the Bracers of Falcon’s Aim are practically "keen for bows".
At 8th level you should be letting quite a few arrows fly. You have 2 attacks one at +8 and one at +3. You might have Manyshot so the +8 attack could be 2 arrows at once (with 1 attack roll). Or you might have Rapid shot for one additional attack (but give you a -2 to all attacks for the round). Do you have a wizard to throw Haste on ya? I have a ranger in my game (with both manyshot and rapid shot) and he destroys encounters….now you are using a small weapon and have a hit to strength that maybe effecting if you can pull your composite bow without penalty, but if your DM has houseruled that you can put keen on the bow…with 3 attack rolls (4 if hasted) and one of those being 2 arrows (from manyshot) you should be keeping pace if not beating the heavy hitters (3 to 4 chances to crit each round). Look at your feats and maybe take rapid and manyshot if you can. Also the ranger spell Gravity Bow will help overcome the small bow size.
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Also the feat Clustered shot just recks encounters. Our Ranger would hit with 5 arrows (hasted, many shot, rapid shot, level 10) and all the damage stacked before any DR. Really helpful. Also taking Improved Precise shot and saving for a seeking Bow means if you know where they are you can hit them. Can't quite shoot around corners but dammed close.
Finally be patient and wait for level 10. Then use instant enemy on every boss fight. You will be out damaging most everyone.
It is from the Advanced Player's Guide(APG).