Thank you! I just tried with a .rtf Mark Twain novel from manybooks.net: it works! I had seen that option but never tried it.The version of CoolReader is 3.0.19 in this April Lbook firmware.
But it was more impressive in Chinese!
It seems their hack consisted in putting a new very clear Chinese font in a Lbook firmware, nothing more.
I expected there was a way to make a whole book in two separate columns, for example the left one a Shakespeare play in English, the right one the translation in another language, or a simplified english version.
I will have to try that with a software to create an ebook. And not a pdf!