Remember that someone reading your CV isn't going to read every word of it, they're likely to scan quickly through it looking for key points. While in Doc/PDF format you can use typographical tools to aid this (font size, line width, layout, etc.) in plain text you have far fewer options.
This of course assumes that you can put together a well designed CV, obviously a plain-text CV is going to be better than a CV written in size 20 comic-sans (I've actually seen this).
PDF is at least a fairly open standard and allows proper formatting with all (reasonable) viewers. If you do use a PDF/doc, make sure you spend a little bit of time working on the formatting of the thing. Or just download a LaTeX template..
Plaintext works fine for reading on a screen, but anything done on paper needs some semblance of typography to it.
Generally speaking I keep a copy of my resume in .tex and .txt format, with appropriate tweaks to each to make it look nice.
If it happens that you send resume directly to the technical person, good chances he won't care or even will be more pleased by plain text.