That is an excellent point. I generally like PDFs because I can zoom them on my large monitor, and they're fine on my iPad. But there are lots and lots and lots of different types of tablets and pads these days. It's only going to get more diverse.I don't want PDF's.
PDF's are useful for printing, but lousy for reading on a smartphone, tablet and so on. Instead I want a digital format like epub where you are ment to change the font size yourself.
This is especially true for modules which would be so much easier to run if they were made in a better format than the age old "page" format. A format where you could flick through the mobs in the encounter and not have them bog down the encounter text with lots of stats.
The epub format really is quite good for multi-platform sales. I can use epubs on my computer / big monitor just as easily as on my smartphone, and they're fine on my tablet as well.
If they can do epubs, without crazy and invasive DRM that's in your face all the time, then definitely go for something like epubs.