[uberman]

For text, 8-bit indexed png or if your scans are high quality then black and white indexed png files will be hard to top.

After the downsample, run them through pngquant or advpng. The latter supports zopfli via insane mode and will likely take the longest to compress with the smallest result.

[hexeater]

Thanks for suggestions. Seems like it is a standard process to convert to grey scale prior to OCR anyway. :-)