Mike Macgirvin
  
There wasn't anything in the Friendica code to specifically render pdf files. You saw some pdfs because scribd was an oohembed/oembed provider and had their own viewer.
Mike Macgirvin
  
Most browsers have native PDF rendering these days (in javascript, not using the buggy Adobe proprietary plugins). If yours does not, there's a link to download the file. From a systems and security viewpoint it's an order of magnitude less annoying than a YouTube or Twitter embed.
Anmol Sharma
  
It happens to me too sometimes. But don't you see the alerts in the notice field ? They tend to be there despite of comments and likes being there in feeds or not.