Does a long bow get 1.5x Dex mod to damage in the same way some melee weapons add 1.5X STR, since it's two handed? I cannot find anything anywhere on the matter whether for or against.
No one gets 1.5x Dex mod or Str mod to any weapon's attack rolls, so I will assume you are thinking damage, since a 1.5x Str bonus applies to two-handed melee weapons. However, no one gets a bonus to damage, Str or Dex, when using projectile weapons in Pathfinder.
It's on Page 141 of the Pathfinder core rulebook I have, under projectile weapons.
No, unless you use a composite weapon or possess a relevant class feature you don't add any attributes into your damage roll with projectile weapons. You still add your strength modifier to the damage dealt by thrown weapons. Even with optimization, there is no way to add 1.5XDex to your ranged damage; all the relevant abilities add your full Dex modifier so the effective multiple will always be a whole number (e.g. 2XDex+Str).