KFax can not (yet) send or receive Fax files. For this I wholeheartedly recommend the mgetty/sendfax ftp://ftp.leo.org/pub/comp/networking/communication/modem/mgetty and hylafax fax ftp://sgi.com/sgi/fax send/receive packages which are available for free.
HylaFAX: http://info-sys.home.vix.com/flexfax/toc.html
sendfax/mgetty: http://www.leo.org/~doering/mgetty/
A "zoomed-in" document can be moved within the KFax view, by clicking and holding down the first mouse button down while moving the pointer.
KFax can not display image file formats which are not related to faxing, such as an arbitrary tiff or gif file.
By default raw fax files are interpreted as g3 files. If you want to display faxg3-2d and faxg4 files you have to specifiy this on the Fax Options dialog. In the case of raw g4 files you might also need to specify the height in scan lines of the fax if it differs from the default 2155.
KFax makes use of the KDE Drag and Drop protocol. This means that you can drag and drop files form the Desktop or kfm or some remote ftp site that you have open in one of kfm's windows onto KFax.