If you get the chance, read Orwell's "Politics and the English Language." Never use a metaphor, simile, or other figure of speech which you are used to seeing in print. Never us a long word where a short one will do. If it is possible to cut a word out, always cut it out. Never use the passive where you can use the active. Never use a foreign phrase, a scientific word, or a jargon word if you can think of an everyday…
And then please read this essay from the Language Log in reply (or as a counter-point, at least): http://languagelog.ldc.upenn.edu/nll/?p=992 .
The same goes for Strunk and White, who in the very passage instructing writers to abjure the passive, say:
"Many a tame sentence of description or exposition can _be made_ lively and emphatic by substituting a transitive in the active voice for some such perfunctory expression as /there is/ or /could be heard/."
Still, at least the guide linked in the OP has picked actual examples of the passive voice as illustrations, and concedes that there are times when it can be reasonably used (sorry, "writers can reasonably use it"). This makes it something of a rarity, even if the advice is still poor.