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 mch prettier and featureful that ghostview's plain old Athena gui.
TeX DeVice Independent files may be previewed under X11 with xdvi. Modern versions of xdvi call ghostscript to render PostScript specials.
A VT100 driver exists as well. It's called dgvt. Tmview
works with Linux and svgalib, if that's all you can do.
Adobe's Acrobat Reader is available for Linux; just download it from their web site http://www.adobe.com/.
You can also use xpdf, which is freeware and comes with source, and I should think Ghostview supports viewing PDF files with gs under X11 by now.