I can read pdf technical drawings and specs on the cybook with the latest firmware & as Pulp says, they are readable, especially with the pan mode. However the Pan mode "jumps" rather than gradually moves as you may expect in a pan, so keeping track of a tech drawing is challenging. It was never really designed for this and some of my pdfs are A3 rather than A4. For A4 drawings etc though it is useable.
The DR1000S which I have as well, does it much much better, bigger screen of course & the pan is gradual rather than a series of large jumps.