Nearly anything you can print can be viewed on the screen, too.
Ghostscript has an X11 driver best used under the management of the PostScript previewer gv. The latest versions of these programs should be able to view PDF files, as well. Note that gv has replaced the older previewer "Ghostview"; the new user interface is much prettier and featureful that ghostview's plain old Athena GUI.
TeX DeVice Independent files may be previewed under X11 withxdvi. Modern versions of xdvi call ghostscript to render PostScript specials.
A VT100 driver exists as well. It's called dgvt. Tmview
works with GNU/Linux and svgalib, if that's all you can do.
Adobe's Acrobat Reader is available for GNU/Linux; just download it from the Adobe web site.
You can also use xpdf, which is free software, and I believegv supports viewing PDF files with gs under X11.