Custom fields for invoice / PDF
Tokens are available only for contact related columns - 
if you want to use contribution related columns as tokens then you need to create an extension and add token hooks for making the values available in tokens list 

Please refer to the following link for creating token hooks

http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_tokens 

http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_tokenValues