RAW, I say this can go two ways, depending if you see the parenthesis as "guidelines" or clarification.
Either you see the parenthesis as inferior to the other text. So the bold text below is the rule for the spell, and the parenthesis is just a rule for which kind the cleric get, which doesn't apply here.
So the rule for the spell for a straight archivist would be "Either DR/Evil or DR/Good." How to decide the type of DR becomes harder here. I would argue to go with alignment, but I do realize this would cause problems with neutral archivists.
You gain damage reduction 3/evil (if you normally channel positive energy) or damage reduction 3/good (if you normally channel negative energy).
Or, you could argue that the parenthesis states the requirement. "If (and only if) you channel positive energy, then you get DR/evil." (And vice versa.)
If you want a definite answer of which of the two above is correct, I think we need a lawyer (or other professional used to study and interpret text). The problem would be that a D&D handbook (unlike a lawbook) probably have no written rules (or laws) on what is valid/takes effect in case of indiscrepancies in the text.
Personally, if I ran in to this kind of problem, I would place it in the hands of the DM with the arguments I made above. Though that wont really help you here. XD
I hope I contributed with something.
/Yirrare