I can download PDFs and view them, but can't view PDFs from my computer?

Just as the title says, I can download PDFs from the internet or emails and view them just fine. If I download the PDF to my desktop, I cannot open it onto my computer for viewing at all. The PDFs still exist, as I can send them to others.

Any idea what could be going on? I have a strong feeling it is user error on my part. lol