16. On-screen previewing of printable things.
Nearly anything you can print can be viewed on the screen, too.
16.1. PostScript
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.
16.2. TeX dvi
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.
16.3. Adobe PDF
Adobe's Acrobat Reader is available for Linux; just download it from the Adobe web site.
You can also use xpdf, which is free software, and I believe gv supports viewing PDF files with gs under X11.