
I suppose we all have come across that
"red button" on websites asking you to download Acrobat Reader to read a PDF document. Even free for download, it still is a proprietary solution and in no way a Free Software tool.
Now, my friends at the
Free Software Foundation Europe have been busy working on a website to propose alternative readers for all OS platforms:
pdfreaders.org. And it seems that there is only one alternative package running on
all operating systems:
Okular 
Another proof that KDE is "The Universal Desktop".
Don't forget to use the buttons provided for your websites:

There are other models in the
"Graphics" part of the website.
Update: if you want to get in touch with the authors of
pdfreaders.org, you can drop them a
mail with this link.