Is "白皮书" a good translation of "open letter"?
No, as stated in your post : 公开信 is a perfectly common 1-to-1 translation for &quot;open letter.&quot;; while &quot;white paper&quot; is a specific term for a type of government-issued document or business to business document, and it is always translated as 白皮书 in Chinese
The function of 白皮书 is different from 公开信. Translating 'open letter' as '白皮书' is a mistake.