what ETL technique should i use for text documents using Hadoop?
I would suggest a data fabric. That would meet your need for data acquisition, preprocessing, data quality, master data management, etc. 

Given I work for Talend, I would suggest our data fabric. =)

Here’s a case study with the Panama Papers. https://www.talend.com/blog/2017/01/17/talend-data-masters-2016-icij-decoded-panama-papers-talend/

The concept in that case study of combining a data fabric with analytic tools is the general concept to use regardless of which data fabric you use.

You can find the trial and open versions of Talend at https://www.talend.com/download/. 

Edit: Here’s another example, which implements a backend and UI showing the ebooks of the Gutenberg project. It allows to import the whole gutenberg index using a camel route. https://github.com/Talend/tesb-rt-se/tree/master/examples/tesb/ebook