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To answer the subject line, without regard for how hard it might be to do this if converting from other formats, the answer is: EPUB and drop the cover image (or replace them with very small versions), and be sure not to embed any fonts unless critical to the content. With epub the cover typically uses more space than the text, even on very long books.
If the books use internal (non-cover) images then care is needed in how you handle these. (Choosing PNG, GIF or JPG as best suits each image can make a big difference - stay away from bitmaps.)
You converted from djvu to PDF - the resulting blow out in size may suggest either a poor tool or bad choice of options. It may have generated images of the pages. Another thing that can really blow out PDF size is choosing Print optimisation options - for on-screen viewing you don't need that, even for printing it can be of dubious value. Conversion is something you have to play with until you get the compromises that best suit your purpose. ... And even so, conversion to PDF would not be my first choice for on-screen reading.
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