I have used pdflrf
to convert DJVU files to LRF in the past. The problem you are facing is that it will render a full page of text as a full page graphic image that will not be readable on a 6" screen.
I have converted a number of books from the Internet Archives to LRF (part of the Harvard Classics
series) and I had to bypass the provided TXT files due to the quality of their OCR. What I did was to edit the PDFs in Adobe Acrobat to remove the header and footer of each page and then convert the resultant file in ABBYY PDF Transformer 2.0. This yeilded a far superior OCR that required perhaps only 10% of the editing time that the Internet Archive TXT files would have required.