I'm all for free papers by the way, nothing is more annoying that researching things and hitting paywalls but someone has got to pay the people doing the publishing work.
Also: If I order a paper from our library or I download it myself, it often comes with an on the fly generated cover page with my IP address on it. One can remove that, certainly but there may be other mechanisms to tag papers. Amazon reportedly investigated (and implemented?) putting specific, unique errors in DRM free ebook copies to identify sources of piracy. So I wouldn't advice you to just send the PDF around unless you are the author maybe and have a PDF that did not go through the publishing process.
Still loving the initiative though ;)