The third image is taken at 2.5 kV and the soft low-density toner produces enough secondary electron contrast to be imaged. An outline of the letter "a" is very clearly visible, and it is obvious that it is constructed of fused blobs of amorphous polymer bonded to the paper's cellulose fibers.
The point of this application note is that the beam energy must be selected depending upon the system to be imaged-- not just in consideration of electron-optical performance of the column. In this case the actual part of the sample to be imaged does not require optimal column performance in terms of probe diameter, but sufficient secondary electron emission is required-- and that is possible only at lower beam energies.
That is the other point of this application note. Not only does the beam energy need to be selected according to sample, but also the imaging detector. In many cases a sample is best suited imaged at several beam energies and using several detectors to fully understand the sample under investigation.