Well, if you're applying to a position with the HR department, I guess you wouldn't.
For the vast majority of people posting here, we're working in departments that have nothing to do with HR, and HR's ability to read PDF files has no bearing on our (potential) boss's ability to read PDF files.
Hum, that's probably true, however stupid HR departments correlate very well with places that will soon turn into a hell to work in. (Unless they are powerless to decide who gets into - but then the CV wouldn't be a problem.)
If one can choose, I guess that is one of the most important signals.
