What are the legalities of letting someone borrow a book that you have purchased that so happens to be in PDF format with no DRM?

I know that I can lend a printed book to someone and this not infringe copyright, but a pdf?

I know that the thought police are going to be on my case about "no one here is a legal expert" or the "question can't be answered" or "controversial", But I would like to have the opportunity to at least surface the issue, and allow people to comment. I am not looking for point scoring on this, or even an answer, but as stackoverflow has a rather large technical savvy audience that would appreciate the question I think that it could be the next hurdle that will stop technical progress.