The Free Software Foundation encourages selling free software. As the Foundation has written, "distributing free software is an opportunity to raise funds for development. Don't waste it!". For example, the FSF's own recommended license (the GNU GPL) states that "[you] may charge any price or no price for each copy that you convey, and you may offer support or warranty protection for a fee."
Adobe created the PDF format, so it stands to reason it would still offer a pretty kick-ass free PDF reader, albeit one that's not as full-featured as some others, since it has other tools it wants to sell you. (The DC stands for Document Cloud, Adobe's attempt to get you to store everything online via Adobe.) Using it, you can easily annotate or sign PDFs, fill out forms, or even save a PDF to Word or TXT format.
Freeflys: Another strong contender in the freebies space, Freeflys has appeared on NBC’s Today Show, The Doctors, Fox News and a handful of other media outlets. Similar to most of these sites, Freeflys utilizes email alerts to tell users about free-sample offers as they happen. The site also allows you to search for a specific product with the handy search bar.