You may only be able to see one page of a fax.
This is down to the viewer you are using and its inability to display multi page tiff images correctly.
There is a free piece of software for Microsoft Windows which can resolve this issue with tiff images. Please download it here:
For MS Windows:
However, for Mac and Linux, .tiff file viewers are built in and found to work.
Please note, it is not possible to change the file format faxes are sent as.