Today I received a docx document via Facebook. I tried to open it on my Mac. I couldn't open it "because it is from an unidentified developer". This surprised me. I've seen such alerts before when I open a newly downloaded program, but never for a document. So I right-clicked, selected "open" and then I got another alert if I was sure, and then I needed to give admin credentials.
Why does this happen? I've never seen this for pictures or PDFs I downloaded. Can you explain why this happens?
As SudoNinja explained in his answer, it's the OSX Gatekeeper functionality that stops executing newly downloaded programs with a warning. The question remains why Gatekeeper catches this file, which I have scanned with Google's VirusUploader.