eix kde-base/kpdf does not support filling in formulars. It's unmaintained, so it's unlikely that the function will be implemented in the future. The solution suggested in kde bugzilla (see link) is to switch to okular, which is unavailable for kde-3.5-users in gentoo. This means a major feature of PDFs is missing. Instead of the formular fields, there are characters like "þÿ" appear. Please add an ebuild for okular for kde-3.5.
Consider me incompetent, it's possible to install the 4.2-ebuild under kde-3.5. Sorry for the inconvenience -_-