Does anyone know how to download all the historical news articles dump(not just headlines) for a ...

Eikon is definitely not the right product for this. Eikon is an end user product and contractually any data retrieved from Eikon may only be utilized for the user's individual use. What you're asking for is ~70K news stories, and this is just from Reuters, not including any 3rd party newswires. This amount of data definitely falls outside the individual user use case. I suggest you consider and discuss with your Thomson Reuters account manager alternative enterprise products designed to deliver such vast amounts of data, e.g. the Machine Readable News proposition.

https://financial.thomsonreuters.com/content/dam/openweb/documents/pdf/financial/exploit-signals-in-news-for-quantitative-strategies.pdf