Official CHM reader in windows doesn't offer control over the font size after IE7. It can be torturous and straining to read ant-like text in high resolution screen. That's where ChmZoomer comes into play. The plug-in lets you zoom-in and zoom-out the font size in CHM freely. So the CHM would be comfortable to read.
Moreover, ChmZoomer gives you handy full page zoom functionality for all other browser-based applications, such as e-Books, RSS Reader and so on. That is just like in IE7/8. The shortcut keys are the same as IE and Firefox. Press [CTRL +] or [CTRL Mouse wheel up] to zoom in. Press [CTRL -] or [CTRL Mouse wheel down] to zoom out. Reset zoom to 100% with Ctrl-0.