Decreases the number of pixels in the document's background layer. Because background objects are generally low-contrast and smooth, you can subsample this layer without affecting their appearance. The background of a 300 dpi document, therefore, can be subsampled to 100 dpi, which corresponds to a value of 3 (300 dpi / 100 dpi = 3). The default option of 3 decreases the size of the output file and ensures that the letters that were not placed in the foreground are still readable. The default value is set to 3.
pdftodjvu
--bg-subsample=5 document.pdf [document.djvu]