It is an appeal to use a free PDF Reader. Well, under Linux and other free systems, there are a lot of them, and they are all mostly good. I actually do not understand why there are still people who prefer the Adobe Reader under Linux. Not only are there a lot of alternatives, they are also mostly much better (faster, easier to use). Few PDFs are not working on them – the ones created with some strange WMF-Tools (M$ for the win) and of course the ones which are encrypted such that explicitly only Adobe Reader can open them. I had this situation exactly twice in my whole life – one time a PDF created with some strange settings from Scientific Workplace, and the other time from a Professor who wasnt allowed to publish parts of its book without encryption. Even commercial pdf-providers usually dont use this, because its basically useless – it is a crude form of DRM, but modern eBook-Formats have much better techniques for that.
Also under Windows, I dont want to use the Adobe Reader, but actually I mostly use (the non-free) Foxit Reader there. The FSE’s list names Evince for Windows – but Evince for Windows was in a Beta-State and I wouldnt have recommended it to normal people. Okular was stable but needed a full-blown KDE-Installation, and KDE for Windows is still no fun. I never tried Sumatra PDF though. I will have to do this.
Well, actually, I dont like PDF much. Many modern PDF-Files are bloated. I liked early versions of PostScript much better. And at the moment, I like djvu very much. At least for ebooks, djvu seems to be a good format. As a comparably simple format, I like SVG. I mean, its bloated with XML-Stuff, but at least the inner structure is simple.
Its a pity that only few pieces of free software work properly under Windows. Windows is still the main platform for most people, and to convince them of free software, it could be a good thing to actually make them work with it under Windows already.