Hey!
PDs - In my opinion there main use is to add % at gallons shop or for trading
They can also be used on this quest for mags, and non rare weapons suitable for Dark Flow / Meteor material and a few heavenly units.
They can also add specials to a select few weapons also listed here.
All info on exchanging PD's for these items can be found here under 'The Phantastic Bazaar'
PT's = Photon Tokens
They can be bought from the quest above 'The Phantastic Bazaar' for 6PDs each, but you can normally buy them from other players for about 5PDs.
You can add hit to any weapon with them (except for s ranks) as long as there is room for hit.
1 PT = 1 % of hit.
PT's can drop from daily crates or certain events.
DTs = Donation Tokens, they can be redeemed with GMs for items (although it is almost always cheaper to buy items from other players on the market forum)
DTs never drop in game, and can only be created by donating to the server. Although you dont have to donate to trade for them with other players (I hope that makes sense)
DTs can be used to add % and hit - for a full list of what you can use DTs for see here:
Exchange rate is typically:
5 PDs : 1 PT
10 PDs : 1 DT
so 2PT : 1 DT
In my experience players will take any currency to trade with.
Hope that helps a bit