Or use less/xless/gless as a viewer instead of printing. You can view PostScript documents with gs and view PDF documents with either xpdf or acroread (from Adobe). Ask yourself, do you --really-- need a hardcopy each time you decide to print something out.
For your own documents, a good idea is to create them in landscape mode, making them easier to read without having to scroll up and down. Otherwise, readers may find reading from the screen inconvenient and may then prefer printing. See PDF editions of the free Tux Magazine for a nice examples.
What are the reasons why people don't read from the monitor:
Reading is slower up to 30 percent, see N.N.. The speed can be improved by better hardware (e.g. TFT display, greater screen, though this is against the other below to take smaller CRTs) and better display software (type-1, t1lib, truetype, freetype).
Paper seems better organizable and more secure to some people. I suppose this can be treated by better software (e.g. Linux) and hardware, too.
Some people use handheld PC (e.g. PalmIII, Newton Message Pad, Psion 5) to carry around documents to read rather than printing them out. At TuxMobil you may get information about Linux compatibility for handheld PCs and PDAs. Or they take eBook readers to read "Electronic Paper". At TuxMobil there is a section about Linux compatibility for eBook readers, too.