... για λογαριασμό των εκδόσεων «Φίλιππος Νάκας»
The question here is of the name εκδόσεις Φίλιππος Νάκας in English. there are two possible approaches: (i) find out what the company calls itself in English, and use that as the name, or (ii) construct an English-sounding name.
The form εκδόσεις Χ is based on the French model of éditions X, regularly used by French publishing houses. Editions X does not work in English, and X editions, is quite unusual. A generic word in English would be publications. the Bank of English includes such names as Conde Nast Publications, British Museum Publications, Plesman Publications, Lonely Planet Publications, and so on.