Share 'What does OPENED really mean?'

Hi all, I have a quick question. I have sent out a total of 4 emails to date. Each has contained links (between 1 and as many as 4) to PDF files I have on my server. I have consistently gotten about a 33% OPEN rate but very few clicks. For example, this morning 49 recipients OPENed my email but one 1 clicked any of the 3 links on my email.... This is my question. What does OPEN really mean? I...